Siegecraft_And_Siege_Warfare

"Siegecraft And Siege Warfare. \- The earlier article on Fortification (10.679) reviews the prevailing ideas of defence against siege-warfare just before the World War. Opinion was then still unsettled on fundamental points, as well as on those differences in arrangement of the available elements of defence which have always divided the military engineering world into two or more schools. In the earlier days, Vauban and his competitors might disagree as to design, but they were in agreement as to purpose; and even at a later date, when the "bastion" school carried on its controversies with the "polygonal " or caponniere school, there was complete unity as to the necessity of permanent fortifications and substantial unity as to their functions. But the economic history of the 19th century, and the military history of its latter half, had brought the principles as well as the practice of fortification into the melting-pot.

Amongst many reasons for this, the following were the more important: (a) The increased size of armies, made possible by the credit system of finance, the universal service system of recruiting, the industrial system which could arm them, and the road and rail system which enabled them to disperse without risk in order to feed on the countryside, or to remain massed without starving through a breakdown of convoys, or both.

This increased size soon reached a point at which the oldfashioned fortress ceased to be an adequate base for the army's depots, or an adequate shelter in which to refit after defeat. There were signs of this even in 1870, although by that date the fortress had expanded into an entrenched camp of large perimeter, and between 1870 and 1914 the scale of field artillery, field transport and field ammunition for a given force, was practically doubled.

(b) The character of war, as between " armed nations," in which, in principle, a speedy decision by battle was sought at all costs, whereas warfare between the old professional armies had been prolonged from campaign to campaign. The objects sought by each side were now rather spiritual than material, or at any rate more general than local; and the fortress, which used to be judged according to the degree of protection it gave to the material objects of enemy desire - a city, a province, a port - came to be judged according to the degree in which it aided or impeded the manoeuvres of a field army seeking to win the war in battle. The task of fortification thus became much less positive and definite, and a programme of works took on a somewhat speculative character.

(c) Development of communications, which, besides the effects referred to under (a), had that of making civilized countries everywhere or nearly everywhere penetrable. The fortress as conceived of in the 18th and early 19th century, therefore, no longer exercised any power of control beyond the range of its guns or the striking radius of its semi-mobile garrison. And it could easily be " turned," and then either enveloped or - by means of a masking force - eliminated as a factor in the campaign. Cases indeed remained, and still remain, of " obligatory points of passage," where local control of the route by means of fortification implies strategic control of the adjacent regions which are limited for their intercommunication to that route. Especially is this true still of rail communication. But in the main, armies and their transport can, in present-day west and central Europe, move where they will, except through areas directly under the tactical control of fortifications.

Further, the rapidity of communication as well as the wealth of routes enables a modern state to concentrate its defensive forces in the threatened region far more rapidly than of old, and the necessity for fixed defences, to gain time for the assembly of mobile forces, steadily declined.

(d) The development of the technique and manufacture of weapons of war, from about 1860, became so rapid that permanent fortifications of any given design were liable, like modern warships, to fall into obsolescence after a brief life of usefulness which contrasted sadly with the long career of a place like the old citadel of Antwerp, built in 1567 and besieged with all the forms and means of siegecraft in 1832.

Three out of four of these operating causes, it will be noted, are extrinsic, and one only intrinsic. In the case of the latter, operating alone, it is easy to conceive of a sort of duel between the gun constructor and the military engineer, analogous to the continued contest of gun versus armour plate. Few fortresses have ever had the good fortune to be fully up to date in design and equipment at the moment of siege. The reply of the French engineer who was asked what he would do if the Germans made the length of their scaling ladders greater than the depth of his ditches, expresses an inevitable condition of permanent fortification design. " It will always be easier," he said, " for the Germans to make scaling ladders than for me to dig ditches." Similarly, it will always be easier to make a new gun that will cut through a given thickness of concrete or armour than to increase the latter. For - questions of expense apart - the fort is a permanent sentry guarding against surprise, and the reconstruction of its works is a heavy piece of engineering which not only takes time but frequently renders them useless for the period of the repairs. Thus, in 1914, war surprised the fortress of Belfort when four of its principal works were under reconstruction. And if, as is generally the case, the programme of reconstruction is so drawn up as to minimize these risks, some part of the fortification system is sure to be obsolescent at any given moment. At any such moment, then, the question is not whether the means of attack have the upper hand - practically this is almost always the case - but whether the superiority is of such an order that the fortress or fort is useless. The new long-ranging powers of siege artillery in 1870, subjecting the area intra macros  to concentric bombardment by an indefinitely numerous attacking artillery, and the demolishing powers of the superheavy siege howitzers evolved in Germany and Austria between 1900 and 1914 - at least as against average concrete - were superiorities of that order.' But such cases are not frequent in military history, and it is more usual - in modern times, especially - to find a sort of thrust and parry, in which the artillery of the attack maintains a lead, but not a decisive lead.

The extrinsic causes in operation, meantime, were tending to bring about radical changes in the very meaning of fortification. Outwardly, the controversies of the period1885-1914turned on technical questions, and chiefly on whether improvised fortifications could be shown to possess a resisting value practically equivalent to that of permanent fortifications. But in reality it was the feeling that the purposes and principles of fortification, Even in this instance, it must be admitted, the event was due in part to faulty designs which were not up to date even when laid down. Thus the new Antwerp forts (see Antwerp) were only built to resist the 21-cm. mortar, although the Japanese had already, under very unfavourable conditions as to communications, managed to employ 28-cm. pieces at Port Arthur. On the other hand Fort Douaumont at Verdun, where the concrete was excellent (1892) and of adequate thickness, resisted a far heavier bombardment even by 42-cm. howitzers.

as they had been understood in the past, no longer responded to the needs of warfare, that produced the multiplicity of designs and proposals for artillery works and infantry works, armoured and unarmoured works, self-contained and mutually interdependent works, and so on, characteristic of the period of unsettlement. If most of this ingenuity remained, as it did, unconvincing, this was due to the fact that there were great general causes at work, of which, in default of war experience, only the existence and not the effects could be seen.

The size of armies steadily increased, all the European continental Powers being drawn into a competition based on the numbers of citizen soldiers who could be conscripted and financially maintained. The power of armament increased also, and with it the possibility of holding a wider front per unit of armed force. The special results of this, from the point of view of fortification and siegecraft, were the extension of perimeters and the thinness with which a circle or arc of investment could now be maintained. But the more general results were the more important. An army developed along a front of some hundreds of miles could no longer be worked by radial lines of communication centring upon one or two ring fortresses. In Napoleonic practice, a stronghold of some sort was always the centre of operations on which the army's movements pivoted; and only that portion of the theoretical base-line, which was in relation with the stronghold in use at the moment, formed what has been called the " effective " base. With the modern extended fronts, on the contrary, the effective base has widened more and more, until it practically coincides with the theoretical base. In other words, each part of a great army has its own lines of communication and its own sources, the connexion between the army line and the base line, or front edge of the home supply area, being a sheaf of more or less parallel routes. Whatever local variations may appear when portions of the system are isolated and examined by themselves, in the ensemble the strategic structure of warfare in civilized countries had become linear.

But the more the front extended, the more difficult it became to collect any considerable force at one part of it for offensive effort. The " parallel battle in all its horror " - unit facing unit' all along the line - was admitted to be the negation of generalship. But whether the dispositions were made a priori or by manoeuvre within the battle, whether envelopment or breakthrough were the method chosen, the parallel battle could only be avoided by reducing the living forces on certain passive or semi-passive parts of the line to a minimum. The small economies that could be effected on these parts by judicious tactical arrangements in the open - though certainly not to be despised - would no longer suffice to give the really considerable superiority of force necessary for decisive victory on the active front. The expedient of economizing force by sacrificing territory had become, under modern conditions of social and economic life, more dangerous than it had ever been before. Recourse was had, therefore, to fortification. It became one of the roles, if not indeed the principal role, of permanent fortification to economize active living forces on the passive fronts - a principle already applied to field fortification within the tactical sphere. As soon as the competition in numbers had set in all over military Europe, we find permanent fortification developing a new tendency to be linear instead of circular in type. The ring fortress becomes a sort of end-redoubt to a long line of forts, usually drawn along some natural barrier. This tendency is shown in the creation of the Meuse line (Verdun-Toul), and the Moselle line (Epinal-Belfort) in France, the line Namur-Huy-Liege in Belgium, the Sereth line in Rumania, the Bobr-Narew-Bug line in northern Poland, and lastly the Diedenhofen-Metz and Molsheim-Strassburg-Istein lines in Germany. In the same way, most of the new ring fortresses that were not so connected by permanent works were so placed as to be keystones of a linear battle-system; conspicuous instances are the systems Lille-La Fere-Laon-Reims-Verdun and Dijon-Langres-Epinal. Of the rest - setting aside the fortresses of eastern Europe where poverty of communications enabled permanent works, as of old, to dominate great areas by dominating a few nerve-centres - we find Antwerp, Copenhagen, Bucharest, Paris, designed as selfsufficing ring-fortresses, but not so much for playing a part in a battle-system as for serving as refuge for an army and a government that for the time being could not maintain its line of battle in the open field. Three of these four were called upon in the World War to play the assigned role and it is significant that not one of them did so successfully. Bucharest was evacuated as the result of an unsuccessful linear battle in the foreground; Antwerp was given up by the Belgian field army and Government as soon as the choice had to be made between standing a siege and continuing the war in the open; while in the crisis before the Marne the evacuation even of Paris was seriously considered.' Two ring-fortresses, Maubeuge and Besancon, were constructed in France in advance of the battle-system, in order apparently to draw upon themselves a part of the invader's effort condensed on the wings, and to control certain nuclei of communications which might otherwise be useful to him as he pressed forward. But the latter task is really that of a barrier-fort, and indeed Maubeuge was in process of being converted into a pure barrier position when war broke out, the old intention of an isolated defence d outrance having been abandoned. The linear systems with end-redoubts, on the other hand, performed in the war all the services for which they had been designed, with the exception of Liege-Namur; and even in this case failure - so far as there was failure - was due not to any vice of principle but to other factors.

The tendency to force a speedy decision in battle at all costs, specially characteristic of citizen armies, could not but reinforce the effects that the size of armies in itself produced upon fortification. With such a tendency on both sides, the initial deployment on each side would inevitably be carried out, wholly or largely, in accordance with an a priori plan of battle. Strategic considerations for the side which had chosen the pure offensive and moral and political considerations for the side which had chosen the defensive-offensive, imposed concentration close up to the frontier, in the first case so as to seize the initiative, and in the second so as to surrender as little of the national territory and resources as possible. Frontier fortification therefore had as its first duty protection of a line or zone of railheads close behind it; and since railway communication is in principle highly sensitive, a system of ring-fortresses at intervals could not give the same protection against sudden raids as linear defences of equal trench-length. But there was further consideration. An a priori scheme of battle, with frontage and not depth as its main characteristic, is liable to require considerable modification when contact has been made and the first serious combats have produced their varied results, and thus a regrouping process begins in the course of the operation itself. In this regrouping, fortification is called on not only to protect the lateral shifting of masses by rail (as for instance the moves of the French IX. and XVIII. Corps from eastern Lorraine to the Ardennes in the middle of Aug. 1914), but also to send away its own local reserves to the area of decision (as in the case of the three French divisions transferred from the Meuse-line front to the Somme at the end of Aug. 1914).

The more penetrable the country, the more pronounced the linear character of the fortifications that must cover it. Not knowing the direction of attack, the defender must either prepare for it at all points of his allotted frontage of influence, or else resign himself to giving up country that ex laypothesi is economically valuable, and manoeuvre in retreat to gain time. The old policy of devastating a deep zone to cover manoeuvre, occasionally practicable when the organization of the state was simple and predominantly agricultural, is almost or wholly inapplicable in an industrial country. In Oct.-Nov. 1914 Hindenburg devastated part of W. Poland as cover for a lateral regrouping. But when it came to including Upper Silesia in the devastation programme, industrial influences promptly intervened to mitigate it. The 1 In the event Paris played the part, originally assigned to LaonLa Fere, of end-redoubt in the battle-system. Antwerp, after capture, was organized by the Germans for the same purpose, viz. to serve as end-redoubt to the Antwerp-Meuse line.

expedient of making even a narrow zone truly impenetrable by means of radical destructions, adopted by the Germans in France in the spring of 1917, requires both time and an elaborate labour programme, neither of which is available in a battle crisis. In some cases, inundations serve the purpose, but even inundations take time to spread. On the Yser front in Oct. 1914, five days elapsed between the order to open the sluices and the creation of an effective barrier thereby. Moreover, in a generally penetrable country the area of decision might turn out to lie in an unexpected direction, and, if so, a system of fortifications designed to protect regrouping by likely lateral routes might prove to be useless for covering those which actually were required to be used. Thus, a speculative element began to come into schemes of fortification. It was no longer possible to justify heavy capital expenditure on works by reference to plain and definite needs. Already it was admitted that if the unexpected happened, it would be necessary to make shift with improvised works; already a considerable body of opinion held that permanent works, if admitted at all, should be similar in general design to field works; and the tendency in these circumstances was inevitably to trust to the latter, which were cheaper, could be built just where they were wanted, and according to many experts were just as good in principle as permanent works, and indeed better than most of the expensive fortifications already in existence.

This underground growth of the linear principle was fostered by another cause. The time-honoured relation of the town and its defences was altered. Under the influence of tradition theory continued to conceive of the fortress as a circular defence round a town. But the town had ceased to interest the military engineer, who now called it the " nucleus." He disposed his ring of defences at such a distance as to protect the nucleus from bombardment, a convention which was imposing a larger and larger perimeter with every improvement in the range of guns, but his works were snow meant primarily to take a share in the operations in the field. That being so, except in such cases as that of Port Arthur, where the nucleus contained establishments regarded as essential to the conduct of the war, it was almost a matter of indifference whether a particular town should be protected or not. The old relation of the town and its walls had been based on the fact that the walls preserved the town from pillage and murder. In the course of time this had changed to a great extent, and in the era of " cabinet wars " it had almost vanished. But the old mediaeval spirit of the towns came to life again in such instances as Zaragoza, Colberg, Venice, and even if the towns-people were indifferent or sullen, the governor could usually resist pressure from them, because he was strong in the conception of his plain duty as a soldier to defend the post entrusted to him. But when the defence perimeter had advanced out of sight of the town, and the enceinte had either been turned into a public garden or retained, demilitarized, as a historical monument only; when, further, in peace-time no military barrier whatever differentiated the defended area from the open country; when, lastly, two generations of railway traffic had destroyed the self-centred economic life of the town and blurred its particularism - then from the point of view of the town it was in much the same position as any undefended town or village in the theatre of war. It might come within the ambit of military operations or it might not; if it did, it might either resist the invader heroically in the manner of Belfort in 1870 - I, or agitate for demilitarization as Lille did in 1914, but the fact that a ring of forts lay out in the country around it had very little influence either way on its conduct as a town. Open towns in modern times have behaved like fortresses of old, and fortresses like open towns of old.

Correspondingly, the position and outlook of the governor has changed. Formerly the town was his charge, and almost his viceroyalty. His troops were his own; organized for sedentary warfare and not for campaigning, they were not at the disposal of a field army which happened to be operating in the neighbourhood, and the town was in practice defended - sometimes with spirit, sometimes feebly - whatever course operations took outside. Up to the very eve of the World War a French fortress governor was responsible to the Government only, and took no orders from the commander-in-chief of the field armies. The era of " cabinet wars " made little difference to this state of things; the population might be indifferent to the war in the towns as in the country, but to the governor and his troops the fortress was still a charge to be defended. Moreover, it was a real base for the armies in the field, in that the stores and supplies for those armies were accumulated in the fortress, and a real strategic aid in that it commanded routes that were obligatory for both sides. But when, in our own times, the governor had become simply the commander of a certain group of forces destined like other forces to take their part in a general scheme of battle; when the area within his defences had to a great extent ceased to be the source of stores and supplies for the field army, and when railways, needing protection at all points and not merely at a focus, became the principal lines of communication, the choice between evacuation and defence came to be governed by larger considerations of strategy. The governor's decisions therefore were assimilated in principle to those of any tactical executant of the strategist's instructions. He might defend or evacuate as a field commander might hold his ground or retire. But the peculiar character of his responsibility was gone. Even in France, the country which has been most tenacious of the fortress tradition, the old regulation, already quoted, was modified in the 1913 " Regulations for the conduct of Higher Formations," which empowered the commander-in-chief to assume control of any fortress and its forces if he thought fit.' On the German side, units made up from fortress garrisons formed quite one-third of the Eastern armies during the first campaigns of 1914 operating sometimes a hundred miles away from their fortress of origin and in the sequel, never returning to it.

In sum, therefore, causes of a general character operating before 1914 produced these tendencies: (a) to divorce fortifications from their nucleus or central town, (b) to make them rather linear than circular in trace, (c) to bring them into conformity with the battle-scheme of the field armies (with de'classement as the alternative), and (d) to construct them as far as possible .according to the principles of field fortification.

The theory of fortification, on the other hand, was still bound by the notion of a nucleus, and unable, therefore, for the moment to employ its stock of ideas and methods to the best advantage. The practical technique of fortification and siegecraft was, meantime, progressing in details; reinforced concrete had come into normal use, armour was improving in quality, the defence had it in its power no less than the attack to profit by developments in the design of quick-firing guns and howitzers of medium calibres. Observation balloons and kites were available, superior to the old spherical types; wireless telegraphy removed some of the dangers of investment and made it possible to coordinate the activity of a besieged garrison with that of a relieving army. The technique of bored mines developed, and trenchmortars and grenades reappeared. The lessons of Port Arthur in matters of detail-tactics and design were assimilated in the various armies. The enormous defensive power of the machinegun was realized and to some extent exploited. It remained to synthesize the application of these elements, old and new, in an art of fortification that responded to the new demands and conditions of warfare.

This art began to take shape with the introduction of the " group " principle. Advocated by several theoretical writers in the period of controversy, it was applied practically, and on a large scale, by the Germans in the celebrated Feste constructed on the Moselle and the Rhine in the last ten years before the 1 It was in virtue of this new regulation that Gallieni's Paris forces were brought under Joffre's command in the battle of the Marne; and in accordance with the spirit of it that Sarrail acted in the same crisis, when, although only an army commander, he sent imperative orders to the governor of Verdun to despatch his mobile reserves to the battle-field of Revigny. The fact that the governor, General Coutanceau, though himself under attack, complied with this requisition instead of standing on his undoubted legal rights, is itself evidence of the changed outlook of the fortress governor in modern warfare. In a somewhat different way, the confused story of the declassenaent of Lille in Aug. 1914 points the same moral.

World War. They may be considered from two points of view: locally, as examples of a type of fortification, and collectively as a defensive ensemble.  The Feste, as its name indicates, is rather a self-contained fortress on a smaller scale than a fort in the old sense. Although it forms with other such works, and with forts or batteries, part of a defensive system which as a whole may be either linear or circular, it contains within its own wire entanglements each of the elements of defence - artillery for counter-battery, artillery for flanking the intervals, and infantry works for the protection of this artillery against a close attack. But it combines them in a way which differentiates it in principle from the types of fortification characteristic of the1873-1903epoch.

In that period there were, broadly, two opposed schools of thought, and a school of compromise. One school, fairly perhaps designated as the French, favoured an arrangement in which the " forts " form the close-defence element and intermediate battery-positions the distant-defence element. The opposite, or Brialmont school, exemplified in the Liege and Namur works (see 10.698-9 for plans), relied on a simple ring of powerful selfcontained forts, each including both these elements. Variations within the respective schools turned chiefly on the use or non-use of armour, some relying upon it for the protection of all defensive weapons, others confining it to the close-defence weapons and yet others excluding it altogether. The compromise school, favoured by Austrian opinion, sought to modify the characters of each type so as to combine them. In all cases, it should be added, the intervals were intended to be garnished in war with an improvised trench system, with its wire, its dugouts, and its machine-gun emplacements.

The Feste, on the contrary, attempts to combine the two elements of defence without modifying either. Full security for the long-range elements is given in principle by dispersing them, equally full security for the close-defence armament by concentration within an obstacle. To add positive or negative protection, armour is introduced wherever necessary, and loose and " provisional " as the forms may seem to the student of earlier fortification, it must not be forgotten that, structurally, every detail of the Feste is a piece of permanent work.

This very warning, however, suggests that it is necessary - more necessary than ever - for the student of fortification, whether practical or theoretical, to find a satisfactory answer to the question: What is it exactly that we require of " permanent " fortification in the tactical sphere?

The role of permanent fortification, it is suggested, is to give to the garrison or defence force a greater degree of security, and to its armament better conditions of employment, than " provisional," 'i.e. heavy field, fortification can give.

To prevent the enemy's guns from obliterating the defences of the front attacked, and thus enabling his infantry to make its way into the defended area, these guns must be counter-battered and (if possible) destroyed, but in any case neutralized as far as practicable. This implies a counter-battery armament on the side of the defence. According as the guns of this armament are exposed to enemy observation or not, they require, or they can dispense with, fighting protection. But in both cases, and especially in the second, they require to be screened against hostile raids or brusque infantry assaults that may develop during this counter-battery phase, emerging perhaps from dead ground close in front.

This protection can be given in the form of an obstacle to the enemy's passage, so serious that a great and organized effort is necessary to reduce it. Such an obstacle may be a deep ditch, or a system of wire entanglements or grilles, or both. Normally, the former is the better obstacle, but except in country already intersected with canals, wet ditches, river-channels, the use of a ditch requires that the armament to be protected shall be grouped very closely. Unless, therefore, the engineer and his Government are prepared to face the expense and provide cover of the solidest kind' the ditch as obstacle is usually excluded, so far as concerns 1 As Col. T. C. Matheson has pointed out, the closer the grouping the denser the material required to protect it.

the protection of what may be called the main armament. The wire or grille, as compared with the ditch, is greatly inferior as an obstacle, but much more readily created, more easily destroyed, but more easily repaired also. Obstacles can be traversed, either after being broken down by bombardment in advance of the assault or by means of scaling ladders and bridges accompanying it. As against destruction by bombardment in advance, the only remedy of the defence is the counter-battery which entirely or partially stops the bombarding guns. But even without such destruction, the obstacle may be overcome by ladders and bridges, wire cutters, petards and other appropriate means, in the course of the assault itself, unless the work of placing these devices is made impossible by the defenders' fire. Hence the obstacle, whether it be ditch or wire, must be protected by a close-defence armament, and nowadays it is generally admitted that this armament must be a specialized organ. But how is this in its turn to be protected against destruction or neutralization at the critical moment? Practically by its own defensive arrangements alone. And thus, in the element designed to guard the obstacle, we reach the alternate unit of fortification upon which the whole system depends, that which in the last analysis ensures for the main armament the power of undisturbed counter-battery (in the case of a fort d'arret of keeping the forbidden area under steady fire).

The close-defence organ, then, has two functions - to protect other elements and to protect itself. The former presents no particular difficulty, and is merely a question of providing the necessary fire-power. But the latter is the critical problem of modern fortification.

If the counter-battery guns are concentrated, as in a fort, and the obstacle is a ditch, then - quite apart from the material cover required for these guns to enable them to fight - material cover is also needed for the close-defence organ, since its position is practically obligatory. But the cover is obtained relatively easily since the weapons covered are sunk to the level of the ditchfloor, and any necessary thickness of protection can be provided over it both on first construction and later.

But such a concentration of counter-battery methods creates large intervals between work and work, and access to the defended area (which with a dispersed main armament is automatically barred by the obstacles defending this and the fire of the organ which protects them) must be prevented by organs in the works so placed as to control the open zone. In some systems reliance has been placed on the counter-battery guns themselves to do this, but modern engineer opinion generally may be said to he opposed to this, since guns which have been engaged in the artillery duel may have been put out of action by the time that they are wanted for close-defence, and even if intact should be wholly absorbed in their proper task. The organ providing ditch defence, by reason of its situation is not as a rule able to undertake control of the open intervals; and in short the only alternatives are small cupolas or traditore, batteries. The former are open to many objections. If built into the same work as the main armament they are almost as much exposed to premature destruction as the latter is 2 and must be provided with fighting protection on the same scale. If mobile, they are exceedingly costly in proportion to the fire-power they develop. For these reasons modern practice generally favours the traditore battery, which is a casemated emplacement (sometimes a cupola) at or near ground level, giving fire only to the flanks and rear of the work, situated in the rear portion of it and protected against bombardment to a great extent by the mass of the work itself.

But, from the nature of its duty, the site of the traditore battery is frequently obligatory, and when it is combined inside the same obstacle with a concentrated counter-battery armament, the needs of the latter as to site may conflict with those of the traditore. In the avoidance of this, perhaps more than in any other 2 The cupolas of this class in the Antwerp forts suffered nearly as severely as those of the main armament, although they were hardly called upon to exercise their special functions, since the infantry attack of the Germans was not pressed into the intervals before the fire of the forts had been beaten down.

single factor, lies the central idea of group-fortification of the Feste type. Two dissimilar elements have to be both protected by the same obstacle and yet spaced some distance apart. But the obstacle (in such conditions mainly wire or grille) itself requires local close-defence. This " ultimate unit " has thus not yet been arrived at. Nevertheless, this ultimate unit, in groupfortification, has only to give short-range protection to the obstacle, and in practice it is an infantry-manned stronghold, designed to give fighting protection to its garrison,' sometimes provided for, its own local safety with a deep ditch and sunken flanking defences, sometimes organized with a fighting parapet frontally commanding an artificial foreground which is wired, but always having as its real function the protection of an obstacle external to itself.

In the case of concentrated main armament, therefore, it would seem that fighting protection for the counter-battery guns, for the traditore batteries, and for sunk ditch defences is required to be designed on such a scale as will enable these elements to defy, actively or passively, the attack guns of the day and the morrow. The same applies to the shelters in which - in the case of group fortification - the garrison of the infantry work is placed in readiness to man the parapets, but not necessarily to these parapets themselves. Further, in proportion as wire replaces the deep ditch, as an obstacle, heavy and expensive work in peace-time is dispensed with.

In the system of deployed main armament, on the other hand, the proportion of permanent work, it would seem, can safely be much less. With modern artillery means, the sites for counterbattery armament are rarely obligatory; observation must be provided for; but the actual position of the guns, and therefore the line of liaison between observation post and guns, are - to a great extent at least - free from limitations of ground. This being so, the close-defence element of the fortifications may be disposed to the best advantage for carrying out its task - that of protecting a system of obstacles suitably placed between the battery zone and the enemy.

In point of permanent work, then, although parts of the battery positions themselves may occasionally require concrete or even armour, concealment of virgin earth, and alternative positions in the great majority of cases afford all necessary protection. For the close-defence guns, on the other hand - the element which must be able to endure at all costs - the chosen positions are often (if not in most cases) obligatory, and full-scale fighting protection must be given. Even so, there being by hypothesis no necessity to develop frontal fire, and the volume of the required lateral protective fire being relatively little, a permanent work which is essentially a traditore battery and nothing else can be both small and well-covered against frontal fire at an expense much less than that of a great self-contained fort. Its own local protection may be either a ditch with sunk defences or an infantry system surrounded by wire, but these auxiliaries, too, would be withdrawn from the crest facing the enemy to positions on the reverse slope. The only case in which it would be necessary for any part of the system to go forward to the crest and front slope would be that in which the artillery observation and command post is combined with the traditore in one work or one enclosed group. In such a case the post in question would undoubtedly require special treatment as regards its own closedefence. But all that in principle is necessary is that the post and its liaisons should be immune.

On the other hand, the security of the main armament against a rush of hostile infantry was far greater when an obstacle defended by fire completely surrounded it, and military engineers were very 10th to impair this security. No doubt, when the obstacle covering the front of the batteries in the deployed order was fully organized, the latter might be considered safe enough for practical purposes so long as the interval-defence remained effectively in action to protect it. But a danger period was foreseen in which the obstacle was not yet fit to perform its function with certainty. The " brusque " or (more accurately) the " abbreviated ' The term " storm-proof," frequently applied to such infantry works, hardly seems to connote their real function.

attack," proposed by the Bavarian General von Sauer, had many supporters; and as the tendency already mentioned, of modern warfare between " armed nations " is to push the line of resistance as nearly up to the frontier as possible, the fortifications of that line were in fact exposed to instant attack. 2 Those of Verdun and Toul were little more than 20 m., the easternmost fort of Liege only 13 m., from the German frontier, while the western Metz forts could be bombarded from French soil. In former days, this would have mattered less, but the growing mobility of heavy artillery - from about 1890 - for the first time made it possible to employ true siege artillery within a few hours of the opening of hostilities. The attacker, on the other hand, naturally had to forego some of the power of hisattacking means in attempting a coup. His truly mobile siege artillery was limited, or supposed to be limited, to the calibre of 21 cm. Heavier pieces though they no longer took weeks or months to arrive in their emplacements, at any rate took days to do so, and by a sort of general agreement (to which however there were exceptions) the situation was met by placing a part of the main armament of the defence - called the safety armament - inside a closed obstacle. Usually it was an existing fort that was adapted to house the safety armament, but sometimes it was included in the design of a new work. The fort thus in practice reverted partially to its old duty of serving as a battery position, while in theory its function had become entirely that of locally protecting a traditore or other interval defence. The distinction between property and accident was no doubt clear to specialists, but the result was that the generality of armies and peoples continued to look upon a fort as their fathers had looked upon it, till the astonishing events of Aug., Sept. and Oct. 1914 so thoroughly undeceived them - too thoroughly, indeed, for in the revulsion, not merely safety-armament guns but even interval-flanking guns were removed from closed works.

In the system of group-fortification, it was naturally much easier to house a safety armament. No element within the ring of wire need cramp any other, or be drawn into the fighting activity of another, or suffer from the shells intended for another.3 Full fighting protection will be necessary, as is always the case with safety armaments, but, as has been noted above, with more room the same safety can be given with less expense.

In sum, therefore, the necessity of compromise on this question of safety armament has caused the dispersed-elements and the concentrated-elements schools to agree upon: (a) the group or Feste principle for interval-flanking elements, obstacles and defence of the same, and safety portion of main armament; (b) the order principle of deployed artillery, with an obstacle covered by flanking fire, for the remainder of the main armament. This, it will be noted, leaves a real liberty for the treatment of particular cases. The proportion of total armament installed as" safety " is whatever the designer chooses to make it in each instance, the Feste being adaptable to any proportioning within reasonable limits fixed by the contour of the ground. A practical check on enclosing an unnecessarily high proportion will always be the expense of giving full fighting protection.

## Examples of Group-Fortification

Types of forts, both main armament forts and others, being described and illustrated in 10.696, 2 To wire a perimeter or frontage of 30 km. to a depth of eight yd. only requires three eight-hour shifts of (in round numbers) 6,000 workers each, as well as mechanical, animal or human transport for about 4,000,000 yd. of barbed wire, weighing 300 tons or so, and 100,000-130,000 stout posts. Other work to be done includes the clearance of the field of fire, the digging of trenches, the construction of shelters (if not in existence already), opening of communications and liaisons, etc. Land which is occupied by a fortress garrison in war rarely belongs to the Government in peace.

3 This can be demonstrated by the "theory of probabilities." Assume a main-armament cupola 16 ft. in diameter, under accurate attack by a gun having a probable error of 60 ft. in range and 3 ft. in line. Calculation shows that this will probably be hit by 7% of the shots fired. Now assume a traditore element having a vulnerable surface on top of 20 ft. from front to rear and 25 ft. laterally. Placed with its front edge 120 ft. behind the centre of the cupola, this will receive 3.62% of the shots aimed at the latter. Placed with the front edge 240 ft. behind, it will be hit by 0 2% of the shots. In other words, at twice the distance it is eighteen times as safe.

it is only necessary here to consider examples of the newer groupfortification. Three forms may be taken, one of which, the Metz form, has been applied on a large scale, while the others, though academic examples, are fully representative of principle.

Common to all, it will be seen, are: (a) a wire obstacle round the whole group, and behind it an infantry trench-position; (b) very large area, equalling that of town and fortifications together in some of the old Vauban fortresses, and six to eight times that of the typical1873-1903fort; (c) batteries, closed and under armour, for the guns of the main armament (or safety armament) irregularly disposed within the wired area.

So far, all are in agreement. But beyond, there are some important differences. Thus, the Metz group, and those proposed by de Mondesir, both possess powerful infantry works with ditches, whereas the Austrian type lacks this element. Again, de Mondesir and the Austrian text-books agree in attaching the greatest importance to the traditore element, remarkably neglected in the Metz works - at least as originally built. Lastly, the Austrian and German engineers tend to place the centre of gravity of the artillery, and even that of the infantry, defence well forward, while the French author puts them as far back as possible, with only observatories and frontal trenches in the forepart of the area.

The Austrian design (fig. 1) as the simplest, is taken first. On the height 130 is an armoured battery P B, containing four 6-in. howitzers in cupolas, with an observation post in a small cupola in the centre.' Between the cupolas are magazines for the storage of 800 rounds per gun. A passage runs along the backs of the cupolas and ammunition rooms, and two barrack rooms are provided at the ends, with other small rooms as offices, etc., in the centre. In the actual design the thickness of the concrete is, in places, less than 2 metres, which is considerably below present-day standards.

On the forward slopes at S, S, S, are small works, combining in each 2 cupolas for quick-firing guns (intended for frontal closedefence, not main-armament work), with an armoured observation post between them and a shelter for infantry and machine-guns in waiting, to man the trench-line against assault. These are built with a roof of about the same thickness as that of the main-armament battery. On the rear slopes are two powerful traditore batter 1. - Austrian Type ies, T T with quick-firing guns (4 to 6 in each, in order to have sufficient for a distributed fire over the interval, in case fog or darkness makes accurate aim impossible). The inner parts of the concrete masses are organized as barracks (U) and magazines. The traditore  is in two tiers, the upper commanding the country outside and the lower sweeping the (wired) bottom of the ditch. Armour is used for the faces of the gun casemates and nearly 3 metres of concrete form the roof. Those parts of the wired ditch not swept by the traditores  are flanked by counterscarp casemates (F F) containing machineguns or pompoms and protected by 2.8 metres of concrete. Tunnels connect the various elements of the group. In this design, which is simple and, owing to the absence of refinements that would not stand bombardment, strong, there are nevertheless some points of weakness, which may be discussed here, not by way of criticism but because they afford convenient illustrations of certain practical points which the engineer cannot ignore.

The whole of the front wire depends for its intimate flanking upon the counterscarp casemates, F F. In such cases it is necessary to protect the backs of the casemates and their communicating galleries from mine attack, by providing the roots of a countermine system at the outset. This was a form of attack which played a considerable role at Port Arthur. It is perhaps the only way of This is a miniature of the gun cupola, with a telescope placed in the port. The development of the periscope now makes the provision of protected command ports much easier than it was at the time of this design.

dealing with counterscarp casemates, but it is an effective one. At Fort Vaux (Verdun) the Germans made their way through a counterscarp casemate into the tunnel system of the fort, and the terrible gallery fighting of Port Arthur repeated itself. But, unlike the Japanese, the Germans had no difficulty in gaining access to the galleries in the first instance, as the French had themselves blown away the backs of the casemates in order to get convenient access to some external trenches. It is noteworthy that in the final stages of refortifying Metz on the group principle, the Germans were careful to provide the foundations of counter-mine systems. In the Austrian design here considered, nothing prevents the development of mine attack on these casemates except the fire of the central battery P B and the batteries, S, S, S, all of which are exposed and liable to neutralization or destruction by counter-battery. The depth of the ditch containing the main wire varies, and no walls exist to make it an important obstacle in itself. The integrity of the obstacle therefore depends purely on the fire of the counterscarp casemates, and - quite apart from the question of mining attack on these - later war experience has shown that there is great risk of the flanking fire being impeded or intercepted by the debris produced by intensive bombardment. This weakness is common to all ditches, and the problem of keeping the field of fire open had not yet been solved in 1921. But it is evident that the longer the ditch, the more chance there is of a heap of debris collecting at some point in it. In any case, it would seem that to attempt ditch protection for the whole perimeter of a group work involves the expenditure of money that might more profitably be devoted to other elements of defence. Another defect seems to be the small number of the infantry shelters, having regard to the time required for the defending infantry to come out and man the parapet. This is the more important, as this design altogether lacks the strong self-contained infantry work which is the kernel of those now to be described. The evolution of Metz as a ring-fortress is dealt with at Io696. Allusion is there made to new works in progress outside the existing perimeter. These were the famous Feste. They were built in succession from 1899 to the outbreak of the World War, and were continued and practically completed in 1915. Their characteristics were only approximately known at that time, but when Metz was retroceded to France by the Treaty of Versailles, not only their present condition, but their history and cost accounts became available. (See the French official Revue du Genie of Jan. - Feb. 1921.) The Feste in fig. 2 (from the Revue du Genie) shows an actual example. It should be understood that the Germans designed the earlier works of this class with a minimum of defensive precautions, notably in respect of external interval flanking, but that, in the later works constructed in the two or three years prior to the World War, there was a marked tendency to develop the hitherto inadequate external flanking, even at the expense of the main armament, which on this line of evolution would, in due course, have become a " safety " armament only. The group-work illustrated is rather of the earlier than of the later kind, as it is lacking in the traditore  element. But it is one of the greatest advantages of the group-work over the cramped fort that additions and alterations can be made as required, and in fact many such works at Metz were provided later with 57-mm. and 77-mm. traditore batteries.

The Feste forms an irregular quadrilateral, measuring, from outer edge to outer edge of wire, 1,200 yd. from front to rear and the same from flank to flank, with an area within the outermost wire of about 120 acres. At the front and rear angles there are strong and minutely organized infantry works, which form the basic units of the system: their rOle is to flank the wired perimeter and to look after their own close-defence as well. At the right and left angles, the perimeter trench takes the form of redoubts, which contain, in their forward sides, infantry observation posts, and, in their rear sides, both observation posts and organs for flanking the rearward wire. In the interior of the Feste, four armoured batteries for main armament are disposed irregularly and each has a war barracks attached, communicating with it by underground passages. The perimeter trench is provided at intervals with armoured sentry posts. The artillery observatories are aligned on the front slope, and have tunnel connexions with their batteries. The fifth battery is a dummy - a device freely used in these Metz works, in which there is plenty of room. The perimeter wire is sunk to a depth of 2 metres, and the ground in which it is bedded is sloped up to the infantry line, which has the lowest command compatible with its functions. This perimeter wire is carried round the main works (oi, 02) also (though partly unflanked), but the strength of these lies in their inner system. Behind the perimeter wire and the advanced parapet or covered way lies a deep ditch (20 ft.), wired at the bottom and provided with a concrete counterscarp. The floor of this ditch is flanked (in the case of the forward work oi) by a double counterscarp casemate at the apex and a small caponniere in the gorge.

About the same time as these Metz works were being evolved in Germany, Lt.-Col. (afterwards General) Piarron de Mondesir, in France, advocated another type of group-fortification, which, though generally of the same class as the Feste, shows some characteristic differences.

De Mondesir's group is in general outline oval, or rather lensshaped, with the curved front towards the enemy and the flattened front towards the defended region. Like the Feste it bestrides the natural crest. Immediately beside the main wire is a continuous infantry parapet, which has at frequent intervals concrete shelters for machine-guns as well as infantry. On the natural crest, a central structure of concrete and armour contains the commanding officer's observation post and two machine-guns in cupolas for the direct 03 2. - Plan of a Metz Feste. References. oi, 02, main infantry works; 03, 04, flank portions of infantry positions organized as " redoubts"; b1, b2, etc., armoured batteries (b5, dummy); ai, a2, etc. war barracks, fi, f2 etc. artillery observatories; hi, h2 etc. counterscarp casemates, caponnieres and the like for flanking perimeter wire or ditch wire; ki, k2, etc., infantry observatories (armoured); m, m, parapet sentry posts; cf, C2 etc. concrete shelters for infantry on duty; i, entrance blockhouse.

sweeping of the foreground, and towards the flanks two somewhat similar structures house each a 75-mm. Q.F. gun, its observation post, and the observation post of one of the main armament batteries mentioned below. Just behind the crest are two large battle batteries," was destined to survive and multiply in the World War. The rear defence of the inner system is provided by the rear portion of the perimeter trench, with its concrete machine-gun shelters and its wire.

The most marked characteristic of this design is the fact that the interior space of the " group " is organized principally for stepby-step close-defence, whereas it is utilized in a Feste for battery sites. The middle foreground is under the fire of two quick-firing guns in cupolas, but the author of the system evidently does not trust to these organs overmuch, for he arranges that they shall be fired into and destroyed by the main-armament guns if captured. The essential element of the first stage of close-defence is the machine-gun detachments in the front trench, which are housed under concrete till the moment of action. The second stage, which begins when the assault has broken into the front trench, is a combination of counter-attack from the great shelters behind the crest with machine-gun fire from the central crest cupolas; to facilitate this counter-attack, the back of the front trench is smoothed to glacis-form. When all this is lost, the inner system with its " infantry batteries " sited well down the reverse slope, has still to be carried before the main-armament or traditore batteries can be reached, and the machine-gun cupolas of the keep not only cooperate in this third stage, but (with the blockhouses attended to earlier) make it difficult for the enemy to make a lodgment even in the fourth and last stage. In all stages after the first, the curved fire of the trenchmortar battery plays a part. In this respect, and in the free use of machine-guns and local counter-attack, de Mondesir's fortification anticipated by ten years or more the trench-warfare methods that developed in the World War.

The above outline account of applications, practical and theoretical, of the new " group " principle requires the addition of a few details as to the principal constituent elements of such works, - the counterscarp casemate or caponniere for low-flanking of a ditch, the mainarmament or safety-armament battery made up of cupolas (as distinct from the cupolas themselves), and the traditore battery.

The Austrian counterscarp casemate, illustrated in fig. 4, is constructionally a simple example. Under the counterscarp wall, on the further side of the ditch, facing the salient angle of the work, a chamber is formed with embrasures for rifle, machine-gun or lightartillery fire along the two adjacent ditch lengths. In this case armour is used for the embrasures, each gun-room (K K) having two very light guns or pompoms. B is a living room for the squads assigned to the defence, A a latrine, St a stairway leading to P, a concrete tunnel under the ditch which communicates with the body of the work.

Fig. 5 shows a counterscarp casemate of more advanced type. It is amongst the most modern examples of such structures, forming part of a 1914 work at Metz. It fires in one direction only. The inner portion of its mass is in ordinary, the outer in reinforced concrete, and the total thickness is 3 metres. Fire is arranged in two tiers, for rifles, and for machine-guns, and one embrasure (the safest) is allotted to a searchlight. The details are worthy of close attention. The top of the wall is formed as an overhang, under which the fronts of the fighting chambers are recessed. This gives enhanced protection from fire, and also from the risk of grenades FIG. 3. - Section of infantry work 01 of fig. 2, on line a-b.

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shelters in concrete for counter-attack infantry. Then, well back, comes an inner system forming a still flatter " lens." This contains the main armament batteries, the traditore element, and the infantry keep. Its front wire traverses the whole interior of the group, leaving the crest elements and counter-attack shelters outside it, and resting its flanks on the infantry parapet, the junction being sealed by blockhouses. Within this wire, which is protected frontally by several " infantry batteries," i.e. loopholed steel screens or penthouses for riflemen, lie, towards the flanks, two batteries of main armament artillery in cupolas and one or two " Bourges casemates " - genuine traditore batteries which can only fire to the flank and rear and are heavily protected and masked towards the front. Centrally placed between the artillery positions is the infantry keep, in this case as in the German the basis unit. It is four-sided and has a deep wired ditch which is flanked not by counterscarp casemates, but by two low caponnieres springing from the base of the escarp at the diagonal angles. In the concrete of this keep are the war barracks of the whole garrison, observation cupolas, and at least two machine-gun cupolas which are in fact the essential defence of the keep. Embryonic counter-mine systems are provided at the salients of the keep. Behind the keep is a battery for small short-range mortars - a novelty which, unlike the present writer's " infantry being thrown in by an assailant overhead. At the foot of the wall is a pit which lowers the floor of the ditch so far that the assailant in the ditch cannot reach the embrasure. This pit also serves to take debris that might otherwise mask the fire of the lower tier. A gallery formed in the mass of the counterscarp connects the casemates of the different angles.

An example of the modern counterscarp is shown in fig. 6 from another Metz work. Here it will be noticed that, for defence and also for ventilation, the gallery possesses a loophole. Over this is a grille to prevent the placing of scaling-ladders and the upper part of the counterscarp wall is formed to a peculiar section which gives a minimum foothold to an assailant scrambling down, and presents an unfavourable striking angle to all projectiles.

The Mass - for it is a mass rather than a wall - is 7 metres thick for 7 metres of height. The communication tunnel between such a gallery and the body of the work (fig. 7) gives 2 metres of (ordinary and reinforced) concrete protection besides that afforded by the earth of the ditch floor.

Some designers, owing to the risk of the backs of counterscarp chambers and galleries being breached by mining, or the communicating tunnel destroyed from above or below, prefer to keep the ditch between the enemy and the flanking organ. In this case a low caponniere is built out into the ditch from the escarp or from the mass of the work; in the work of of fig. 2 the ends of the twostory concrete barrack are arranged to act as caponnieres.  An unusual flanking organ designed for Metz is shown in fig. 8. Here the difficulty of giving sufficiently thick protection for an organ flanking wire at or near ground level (e.g. the outer wire of a Feste)  is met by providing a a c sort of detached caponniere in the form of a low, fixed, armourstructure bedded in concrete, the guardroom, etc., being formed in the mass of the latter. It is intended for rifle and pistol fire through loopholes, and like all modern German flanking organs it has a small searchlight.

A battery for main c armament is substantially an assemblage of two to four individual gunorhowitzer cupolas in line within one mass of concrete, with the space available between and behind cupolas formed into expense-magazines, shell-rooms, d duty men's rooms, offices, etc. Being as FIG. 5. - Counterscarp Casemates.

a rule dispersed over the open ground comprised in the " group " they require subterranean communications with each other and with their observation posts. The latter are sometimes included in the same concrete mass with the gun cupolas, but it is more usual to withdraw the battery mass behind the crest and to push the observatory forward. Batteries are often wired in, and sometimes given means of local protection against surprise attack. They contain not only a large stock of ammunition, but also, nowadays, laboratories and workshops.

+1.00 FIG. 6. - Counterscarp from Metz.

Traditore batteries, by hypothesis, fire only to the flank and rear. They are thus always placed, so to say, behind a corner; that is, protection is accumulated in front of the gun casemates, and this protection is continued laterally for such a distance that a projectile from any likely direction will either meet the covering mass or pass clear of the gun muzzles. The original form is that designed in France and known as the " Bourges casemate " (fig. 9) from de Mondesir's Fortification Cuirassee.  ' / //W / / e ' /// W/ / ^; i/ / ??? ? i i i ? ?? ?/??? ? i ii i ? y i i? °?/T??? ? n?/ ?i. ??? ii n? '?

. - ... :;?..:??.., ...,, -.. ... v.?r FIG. 7. - Communication Tunnel to Counterscarp.

A larger traditore of Austrian design is shown in fig. to. This is formed in the end of a concrete mass of which the remainder contains the war barracks, storehouses, etc. The lower embrasures flank the floor of the fort ditch, the upper - the real traditore battery - sweep the flanks and rear of the external intervals. (T T are the gun casemates; B St is the post of the officer controlling the fire; M M the ammunition room; G S a small flanking element to control the ditch just under the traditore.) Armour is used to reduce the thickness of concrete, both vertically and horizontally.

The gun usually employed in traditore batteries is a field gun of about 75-mm., or a gun of the small naval or tank class, 57-mm., being the commonest calibre. Pompoms (t-pdr. Maxims) and even machine-guns are also found, but as a rule it is the field artillery type of effect that is required and provided for. Cupolas are sometimes, but rarely, employed, as their characteristic virtue of allround fire is not wanted.

## Administrative Arrangements

Any work that has to act in isolation, or semi-isolation, is provided with all the necessary "services." Store-rooms, hospitals, barrack-rooms, etc., with all .;:. =: .._. ,.: .;.  .

? ??': ??'??????? ? ///:: 1 c-d +1.50 .::i:` a;o?,??,,qYAIESIMI ao±511=?o.??<,3 ? e??.` .., e,e  furniture and equipment, must be formed in the interior of the available concrete masses; water must be laid on, lighting and heating provided for. Though hitherto it has not been thought desirable to provide power for working cupolas, etc., it has already become necessary to include a power-room (dynamos and Diesel engines) in the equipment of the Feste.  either by embrasures, cupolas, seatings or fissures, while pure air must be supplied and foul air evacuated somehow. It may be, indeed, that the question of aeration, hitherto subsidiary, will become one directly affecting the fighting design.

## Materials

Earth was always the common property of permanent and of field fortification. Before 1914 concrete and armour were reserved for permanent work, but during the World War both came into use for heavy field works as well. Further, as both in permanent and field work care is taken nowadays to disturb the natural earth as little as possible, and tunnelling freely employed, relief being kept down to a minimum, there remain only, as specially characteristic of permanent work, (a) the heavily armoured, deeply-sunk, mechanically highly finished gun-mounting, and (b) the great concrete mass. It is not unreasonable to consider the necessity, or otherwise, of these elements in a fortification scheme as a criterion of the necessity of " permanent fortification," using the words here in the technical sense for fortification of a kind that can only be carried out at leisure in peace conditions.

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FIG. 8. - Flanking Organ (Metz).

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Amongst these auxiliary arrangements, there is one, however, which during the World War proved to be of predominant importance - ventilation. The fall of the forts of Liege and Antwerp was largely due to the ventilation arrangements proving inadequate. Poisonous fumes from the burst shell penetrated the concrete fissured by its explosion, and filled the underground galleries of the fort, asphyxiating or disabling a large proportion of the already overstrained garrison. In the latter part of the war, the development of gas warfare added a further complication, as it became unsafe to draw in air from outside. The solution adopted by the French for the Verdun forts (which, it should be noted, no longer contained either main-armament or traditore  artillery, but had become pure close-defence infantry works) was to deliver filtered air from a considerable distance, under slight pressure, to certain rooms in the fort, which were sealed against local air by air-tight doors or otherwise. All men working elsewhere than in these rooms wore gas-masks. Although the group principle of fortification, by dispersing the elements of defence, ipso facto dispenses with the subterranean labyrinths of the Belgian forts, and so reduces the effects of explosion fumes, the fact remains that the men in the armoured batteries and the traditore and flanking organs, must be protected from gas entering IBM The justification of the elaborate gun-mounting under armour (the cupola, according to present practice) lies in its power to resist, for an indefinite time, counter-battery by the most powerful guns available. That power was tested, during the World War, above all in the case of the Belgian fortresses and at Verdun. The gross results of these two trials were diametrically opposite - the Belgian cupolas failed, usually through the failure of their surroundings but sometimes by their own defect, while the Verdun cupolas held out magnificently. Influenced by the general distrust of permanent work which the Belgian failures had produced, the French had ceased to rely upon their forts for the flanking of the intervals, and the traditores had been actually disarmed before the attack of Feb. 21 1916. As this was the principal role allotted to the forts, the action of the few distantdefence guns in cupolas (safety-armament) could, ex hypotlzesi, be only a small part of the total artillery defence. But their powers of passive resistance were presumably tested as thoroughly as if they had been the protagonists of the main-armament, and practically none, save some minor cupolas for observation and machine-guns, were disabled at the close of the great siege. Yet they had endured an unexampled bombardment. Fort Vacherauville, for instance, was hit by no less than 2,250 shells, of 28 -cm. and above - two such shell for every three square metres of sur Plan ?l '= ?, f _ I_' ' = - - ? - _? i_; i, ?1 - - - -- --- ----?-..: face. Monlamville received 330 hits by 42-cm. shell. The more exposed forts, such as Douaumont and Souville, deluged with medium and field as well as heavy shell, received over 30,000 hits apiece. Thus, cupolas of the thickness and quality of those of Verdun are capable of all necessary resistance.

.

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pI ./ '9 FIG. io. - Austrian Traditore.

Even more marked is the discrepancy between the behaviour of concrete at Liege and Antwerp and its behaviour at Verdun; as in the case of armoured gun mountings, the Belgian sieges brought concrete into discredit. Here, however, it is a question of quality of material and of conscientious painstaking work. The best concrete has a resistance such that a test bar will break across the stones rather than the cement; the worst is feebler than virgin earth, especially rooty earth. The Verdun concrete had not only been well made, but had also been given an adequate safety margin of thickness; and it was found that, in spite of the volcanic bombardment, 3 metres of special concrete or 2 metres of reinforced concrete, or 2 2 metres of special concrete (or I. 7 5 metres reinforced) disposed as an apron overlying an old masonry vault, with a sand cushion interposed, would steadily resist calibres up to 42-cm. inclusive. Although the effects of these monster mine-shell were felt in the ground to a depth of 14 metres (nearly 47 feet), the fissuring which had ruined the Belgian forts does not seem to have occurred at Verdun.

In the Metz Feste the characteristic disposition is a mass of ordinary concrete with a thick outer casing of ferro-concrete. An arrangement which is favoured by some engineers is to dispose reinforced concrete in layers separated by air-spaces; this had not in 1921 been subjected to the test of siege. The lesson of Verdun, however, is held to be not so much that it proved 3 metres to be a sufficient thickness, but that it demonstrated the importance of mass as a material element of resistance and depth  as a moral element. The smaller concrete masses suffered under bombardment much more severely in proportion than the larger, and it is considered therefore that organs small in themselves such a caponnieres should form part of a larger body of concrete, as in fig. 2, or, or in fig. 10. As regards depth, it was found that in tunnelled habitations well below the level of the forts - the foundations of which at Verdun are in rock - moral remained unimpaired by the heaviest bombardment. At Antwerp, from the nature of its site, no such burrowing was practicable, and the moral of the defenders was subjected to a fresh strain at the explosion of every monster shell.

In sum, armour and concrete in masses calculated according to the lower standards of a previous epoch, but given a safety .margin in application, proved capable of meeting the heaviest strain of bombardment which history records. The problem of the engineer to-day is to determine what safety margin is required for new work or reconstruction calculated according to the standards of Verdun.

A fixed resistance, in situ, of several months, then, is practicable with the materials and means of permanent fortification. That it is not so with those of field fortification is shown by the various trench-warfare offensives of 1916-7. At Verdun the German attack progressed rapidly till it confronted a close-defence system which had permanent works as its backbone. At the Somme, though the Allies' progress was slow, it was sure, and the defense finally evolved a system of " elastic " or " coilspring " resistance, which by its very nature implied the giving-up of ground until an organized counter-attack could be mounted for its recapture. The success of this type of defence, so long as the moral of the defending troops, and their skill in group, platoon and company tactics, remain high, is a matter of history. But it is equally a matter of history that there are positions which, strategically or morally, must be defended to the end without yielding ground, on which the defender must say, with Petain at Verdun, " On ne passera pas." And it is for these cases, so far as they can be foreseen in peace, that permanent fortification exists.

The very words of Petain's defiance, however, carry the implication of a linear system. Fournier at Maubeuge, Kusmanek of Przemysl - governors of ring-fortresses - could not say that the enemy " shall not pass," for the enemy could go round. It was not Verdun the ring-fortress, but Verdun the end-redoubt of the fortified line Verdun-Toul, that compelled the Germans to traverse Belgium in August 1914. In 1916 Falkenhayn hesitated between Belfort and Verdun as the objectives for his attack. Both had been ring-fortresses, but both were now key-points in a long line of battle, and derived their whole strategic importance from that fact. At Liege, we find the ring-fortress splitting itself into two halves which acted to the best of their ability as successive barrier positions. At Antwerp the attacks on the defence were purely frontal; and indeed, save Przemysl, not one ringfortress attacked in the World War was vigorously invested and attacked by convergent radial efforts. But it is perhaps Metz which affords the clearest illustration of the modern tendency to convert the stronghold into the barrier. From 1899 onward the old ring becomes of less and less importance, and the Feste were not only disposed on a wider perimeter, but collectively took the form of a sort of a parabola instead of that of a circle or ellipse. Facing westward, the system began with a group of Feste, forming an end-redoubt; and at the same time the pivot of the field armies that were to swing through Belgium, at Diedenhofen (Thionville), then ran southward along the Moselle to the neighbourhood of Metz, where it curved westward, passed round the S. of the old ring, and E. of the Moselle, took a new direction north-eastward, till finally it was completed by the " Nied position " (created on mobilization) which connected it to the stable line of battle that would be formed by the middle Saar. The southern bend of the system is about 14 m., the westward-facing limb 36 m. and the south-eastward-facing limb about 38 m. in frontage, the whole differing in scale rather than in principle from the familiar " Western Front " of the trench-warfare years which constituted between the Oise and the Moselle a great salient with the apex at Verdun. Outside this blunted redan, Diedenhofen-Metz-Rehlingen, the Germans were prepared to yield ground, but on it the defence was meant to be absolutely stable. The growth of the linear tendency is discernible even in the creation of the successive parts of the system. Beginning with the endeavours to increase the perimeter of Metz itself, the Germans presently bore away at a tangent on each side of the southern sector. The individual Feste became longer in proportion to depth, and finally, in the case of the Amanvillers-Horimont works, of 1912, attained a frontage of nearly 2 m. and consisted substantially of a great ditch with its flanking-organs.

Correspondingly, a change follows in ideas as to interval organization. It is evident that, in a ring-fortress of the1873-1903type, the individual " forts " occupy only a small proportion, ? T $oo ?

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B V of the perimeter as compared with group-works. The latter,, therefore, allow of an extended perimeter for the same admissible intervals. Presently, as we have seen in the case of Metz, the perimeter attains an enormous length, and the ring opens out on the home side, becoming a gigantic redan or salient which comprises a whole region. The standard interval between work and work continues to be honoured at and near the exposed apex, but the gap increases towards the rear till near the root of the salient permanent work may give place to field work and simple utilization of natural obstacles, as in the " Nied position." In the early 10th century, the ring-fortress was practically an enlarged enceinte-fortress with the curtains omitted. Later, though perimeters increased (in order to prevent bombardment of the " nucleus "), the forts still contain the whole of the main armament and retain responsibility for the defence of the interval against methodical attack. The interval is organized according to the needs of minor tactics, so as to prevent the enemy penetrating to the nucleus by any method but regular, large-scale attack. At the next stage (which is the most important in the evolution of ideas) the main artillery (except the safety-armament) is taken out of the forts and deployed in the intervals, the forts becoming " strong points " on a line of battle which has to protect this artillery. It is this stage which is represented in fig. II, taken indirectly from a semi-official Russian work of 1913, by Colonels Golyenkin and Yakovliev (which, incidentally, marks the acceptance in Russia, after long opposition, of the principle of armour protection).

FIG. I r. - Russian Fort, close-defence.

The " fort " is a large and elaborate close-defence work designed specially for the sweeping of the intervals by traditore  batteries, immunity of these being secured by a full equipment of flanked ditches, quick-firing guns under armour and fighting shelters. The arc of fire of the traditores is sometimes carried unusually far forward, as it is laid down in principle that special flanking guns, in or out of the fort, must cover the ground from yards in front to loo yards behind the next fort. No part of the main-armament is included in the fort.

From fort to fort, the interval, it is laid down, should not exceed about 4, 500 yards. Otherwise, the responsibility placed on the traditores is too heavy (the average interval of Verdun, it may be remarked, is only about half this), and closed infantry work (Zw. in the plan) may sometimes be required to command some part of the interval that is not well covered by the forts. Frontally, the interval is held by a number of separate trenchpositions formed with glacis foregrounds (V. Gl.) and provided with concrete fighting shelters (U.) and sometimes barracks (J.). Some of these positions have their own light quick-firing guns (Gst.) for flank protection. Four to five hundred yards in rear of this guarded obstacle are armoured batteries (H.P.B., K.P.B.), wired in, which contain the " safety " part of the mainarmament with its ammunition. For the rest of the main-armament, battery positions (D.W.) with (M.R.) shelters for detachments on duty (A.U.) and protected barracks for those in waiting (A.K.) are prepared 600-800 yd. behind the front line and connected up as required by trenches. For other batteries, told off as a mobile reserve, positions are marked out (O.B.). The main magazines of the sector are shown at M.M. and barracks for the third relief of artillery at A.K. in rear.

Apart from the batteries of the safety-armament and occasional foreground-flanking guns not comprised in the forts, peace-time work on the intervals is limited in this example to the construction of fighting shelters, barracks, magazines and the walls of battery positions. But, as soon as permanent work of any sort appears in the intervals, these begin to lose their blankness, and it is a short step to the modern conception of position-warfarethe attack and defence of a long, shallow zone which contains in its forepart the close-defence and in its rear portion the distantdefence elements. In this zone, permanent work will, we may predict, be called upon to perform the services which field and heavy field work cannot perform. It will provide, generally, those of the essentials which cannot be improvised (such as the concreted or armoured infantry, machine-gun and flanking-gun positions, required in the more exposed sites), and in key sections, those which will make the local defence as nearly as possible inexpugnable. And since, as explained earlier, these key sections can be in many or most cases predetermined, whereas the exact position of the rest may vary according to the opponent's plan of campaign, the greater part, if not the whole, of the material protection obtainable with a given expense will tend to be concentrated in them. Thus they may continue to be labelled with the names of towns or other localities as of old. But, essentially, they will be planned in relation to things outside and not to things inside, themselves. They will secure, or deny to the enemy, the use of certain ground or certain communications, definitely and completely. Locally, in order that the close-defence component may fully secure the operation of the distant-defence component, the permanently fortified key section may form a ring, or possess turned-back flanks of considerable length. But in the ensemble, and in many cases even locally, the tendency will be to reduce the depth and increase the frontage of permanent fortifications, so that the works of standard strength which can be built with the available funds may offer the widest direct and positive barrier to an enemy.

New Weapons in Siegecraft. - The evolution, during the World War, of weapons essentially unlike those for which the permanent fortification above-outlined was designed to use and to meet, raises the question - what modifications, if any, do these new weapons impose?

If the conclusion reached above be correct - that permanent fortification has now become, from general causes, the backbone of position-warfare, instead of being a thing apart - it follows that the new weapons will affect it in any case through their influence on position-warfare generally. Were it the case, for instance, that aerial warfare totally superseded land warfare, the ring-fortress might return in a new form as the means of protection of flying bases. But such speculations would serve no useful purpose here, and it is preferable to confine ourselves to the modifications of detail to be expected in permanent fortification as such.

Chemical Warfare is a term covering several very different weapons. Smoke which " blankets " the defence organization, gas which disables the defenders, vapour-producers which infect areas, are all tactically dissimilar. The strength of chemical warfare as against fixed defences lies in their fixity, its weakness in the fact that anti-gas protection is not limited, as it is in field warfare, to apparatus and devices that can stand rough usage and be carried without inconvenience by the already burdened soldier. One form of anti-gas defence has been mentioned above - the supply of pure air under slight pressure to rooms sealed from contact with local air. Improvements on this line of progress would include more and more of the interior chambers of a work in the protected space, while other devices made the gun embrasures as gas-tight as possible. The most important defence against gas is the impermeability of the structure of the work itself, which practically means its solidity under bombardment. The direct influence of aircraft on permanent fortification is not easy to estimate. There is no limit to the power and flexibility of anti-aircraft guns sited in fixed defences. All fighting elements in the works are fully protected against the attack of super-heavy artillery, and a fortiori against that of air-bombs. The attack of traditore or caponniere embrasures by machine-gun fire from low-flying planes, if practicable, could be --4.3 Ft.IV. ?' f3E3Gl ? ?-- - -__.? -?

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easily countered. Direct attack from the air, therefore, is not likely to affect design seriously. But observation from the air is another matter. Local superiority in the air may pass, in the course of the operations, back and forth several times, and design has to provide against the unfavourable case. It may be assumed with certainty that, in prolonged operations (such as by hypothesis the reduction of a permanently fortified front would entail) every battery position of the defence would be photographed, whether inside a group or fort or placed in the " interval." The defence is thereby compelled either to change position very frequently, or to build in. In the latter case, either the guns must have full fighting protection, or must be so buried and camouflaged as to be practically invisible to eye and to camera at a height of, say 15,000 ft., even when in action, while at the same time provision must be made for confusing the enemy's sound-ranging records. Here, then, the engineer is in the hands of the artillery researcher. Should the latter not succeed in producing gun-mountings, ammunition, and sound-camouflage (so to call it), which can defy the air-observer and the sound-ranging instrument, the engineer will be compelled to put his main armament again under concrete and armour, and perhaps to concentrate it somewhat. The alternative of frequent changes of position has many disadvantages, chief of which perhaps is the probability of having to traverse areas systematically infected by mustard gas.

Armoured Vehicles in the World War were of two classes, the slow-moving cross-country tank and the armoured car. It seems probable that these will fuse into one class, the fast tank. The tank was originally designed for traversing and crushing wire entanglements, and it is in this capacity that permanent fortification is chiefly concerned with it, for the defenders of solid permanent works are practically immune from its actual attack. The question for the engineer is - will the present types of obstacle-flanking organ succeed in dealing with the tank, and if not, how must they be modified? One type of obstacle, the deep ditch with vertical counterscarp, is impossible for the unprepared tank, but such ditches are liable to be bridged at ground level by suitable girders carried by a " bridging " tank; and the best safeguard of permanent works against invasion by tanks is the fact that the latter must traverse a belt of ground which is swept at close range by an organ specialized for that duty alone, and invulnerable to the tank's guns. Although the dimensions of tanks will doubtless be taken into account in planning ditches, the first care of the designer will be to provide flanking pieces sufficiently powerful to destroy the tank as it crosses the line of fire, and to provide flanking fire in such a form that a disabled tank will not mask it. More generally, permanent works which form a section of a long battle zone will be organized to participate in the anti-tank defence of the system, and for this duty the traditore batteries are naturally the most appropriate organ.

The armoured car, or fast light tank, owing to its ability to make an inroad from a great distance at the shortest notice, may prove very dangerous to a fortified area which is in the crisis of mobilization and war armament. Special measures to deal with this risk could never be omitted; so far as fortification is concerned it would seem necessary to maintain at all times a clear field of fire for, and a state of readiness in, the traditore batteries, as well as in such gun positions of the safety armament as command obligatory or probable points of passage in the foreground.

## Field and Heavy Field Fortification

The methods of field fortification in vogue at the outbreak of the World War (see 10.719) were concerned essentially with light fortification, that is, incidental fortification on the battlefield. Starting from the conception of an advance brought to a standstill or a retreating army turning to fight, the engineer gave the infantry man types of works which were defensible after an hour's labour and which could be improved gradually during a more prolonged stay in the position. Lateral and overhead cover were provided, but only against rifle, machinegun and the lightest artillery fire. Great stress was laid upon concealment, obtained by moulding the work as closely as possible to the ground, turfing exposed soil, keeping relief as low as conditions of effect allowed, and also upon communications; a system of short trench lengths was preferred to long continuous lines, which entailed unnecessary labour and assisted the enemy to locate the defences.

A good field of fire up to 400 yd. from the rifles was considered essential. Pivotal positions, whether localities, woods or hills, were organized for all-round defence. These principles, so long as the general course of the operations retained the foreseen open-warfare character, substantially justified themselves; perhaps, indeed, they justified themselves only too well, since it was the great strength of even lightly fortified fronts which compelled each side, in France, to extend ever further and further to a flank in the hope of turning the opponent's frontal defence, and so in the end produced a trenchline that was continuous from the Swiss frontier to the English Channel. The deadlock which ensued marked the beginning of a positionwarfare which was locally indistinguishable from siege warfare as that term was understood in 1914. Nor was it only from the point of view of local, minor tactics that the analogy of siege operations held good. The succeeding campaigns were indeed a siege of the east front of Germany, whose western offensives of 1916-8 have sometimes been described by the Germans themselves as sorties from the Festung Frankreich. For the Allies, as against these xxx11.

offensives, the nature of the defence, especially after the middle of 1916, resembled that of an investing cordon attacked by a desperate sortie, and fighting to gain time for the arrival of forces from other parts to drive in the prisoners and bolt the door again, - though it is true that the siege was mutual and the " sortie " was at the same time a penetrating assault.

Regarded from this point of view, the principal factors which, directly or indirectly, governed revolution were: (a) a theatre of war richly provided with communications; (b) highly developed industrial resources at the back of both sides; (c) enormous numbers of men and therefore labour power; (d) an initial situation of tactical deadlock in positions not deliberately selected but representing only the line on which offensive effort in open warfare had expired; (e) the haunting idea of a break-through, followed by exploitation in open-warfare in the old style.

Taking the technical effects of (d) and (e) together, it was inevitable that positions naturally unsound should be maintained by an equilibrium of forces acting in opposite directions. The attacker of the moment was necessarily attracted by these weak sectors, while the side which intended to attack in the future had a strong motive for holding as forward a position as possible. Fortification was called upon to make these intrinsically unsound positions safe, and all its resources in design and labour were taxed to the utmost to meet the call.

Taking (a), (b) and (c) together, we find these factors enabling each side to produce, to bring up and to apply in situ, technical stores on a scale hitherto undreamed of in siege warfare.

Taking the factors singly (a), by enabling artillery to be collected in enormous strength for an attack, compelled field fortification to develop unimagined powers of passive resistance, and by facilitating the concentration of great attack masses, induced commanders to strip non-offensive sectors of every human defender who could be spared, thereby forcing fortification to adopt forms which required a minimum garrison. By (b) there were brought into current field use not only materials hitherto reserved for permanent work, e.g.  concrete, but also novel scientific devices and weapons of war, such as the air camera, gas, tanks, and, above all, improved methods of production, which increased the quantities of guns, ammunition and machine-guns available per unit of living force. By (c) trenches and obstacle-lines were multiplied to such an extent that there were eventually 20 to 25 miles of trench-work for every linear mile of frontage. Through (d), in the first year of trench-warfare, numerous bitter fights were engaged in on all parts of the battle-front for minor rectifications of position; in these fights there were developed new forms of detail tactics, with corresponding demands upon rear resources for a varied armament for the front-line combatant, whose equipment finally comprised, besides rifle and bayonet, grenades (hand and rifle), the light and medium trench mortars, flame throwers, periscopic sniper-rifles, knives, clubs, helmets and body-armour, not to mention gas masks. It led, further, to the retention of waterlogged trenches, and so to the introduction of pumping apparatus, trench-flooring, and other bulky non-combatant stores, into the front line. Through (c) it was demanded of fortification that it should protect both the masses of men assembled for attack and exposed to enemy " counter-preparation," and also the minimized garrisons of other parts of the front. These factors, operating, singly or together, with different relative intensity at different times, caused position-warfare to evolve, between 1914 and 1918, through three distinct phases, which may be distinguished by the titles field-fortification, position-fortification, and zonefortification. The first of these characterized the western front from the end of the battle of the Marne to about the end of 1915, and the eastern front from the beginning of stabilization (which occurred at different periods on different parts of the front) to July 1915, and then again from Oct. 15 1915 to the end of the war on that front. On the eastern, Salonika, and extra-European fronts, evolution did not pass beyond this stage, because in these theatres the more important of the factors above enumerated were more or less inoperative. The second stage, position-fortification, is that of the Battles of Verdun, the Somme and the Aisne and the Artois battles of 1917; also of Asiago and the last six of the Isonzo battles on the Italian front. The third stage, zone-fortification, is characteristic of the Flanders battles of the latter half of 1917 and of all the 1918 battles. These indications of date do not of course imply that the changes specified came into force, formally and simultaneously, at any specific time. They are landmarks in the evolutionary process, which in this, as in other things, does not tolerate leaps.

In the first period fortification devotes itself, substantially, to the improvement and multiplication of the trenches bequeathed by the expiring offensives of 1914. The trenches are deepened, kept narrow, provided with more numerous traverses and shelters, though the protection afforded by the last-named only with difficulty manages to keep up with the increasing volume of fire from the enemy's medium howitzers. The close ranges at which the 1914 offensives died out have left the opposed front lines separated by a very narrow " no man's land," which is heavily wired by both parties. Concealment, as understood before the war, has thus lost its significance, more especially as all trench systems are regularly photographed from the air. Detail fighting produces the need of circulation and supervision trenches and other parallels behind 482 Sienkiewicz Sierra Leone P, " the trenchy," and calls for more numerous communicationtrenches, to which there is no further objection on the score either of concealment or economy of labour. In plan, the general result is an irregular skein of trenches, from 50 to 150 yards across, following the contour of the front line, which itself undulates considerably and at certain points forms sharp cusps or salients of local importance. The closing-in of key positions for all-round defence, which figures in pre-war fortification, has become more thorough, and internal subdivision has converted these wired-in areas into the " Labyrinth " of Neuville-St. Vaast, the " Hohenzollern Redoubt " of Hulluch and the " Trapeze" of the Champagne battlefield. At the same time the interpretation of what constitutes a " key " position is different; it is chosen in relation to the trench system rather than in relation to the ground. Mining plays no small part in the fight for the possession of particular hillocks, or trench lengths, with a view to improved observation or better defence. But from the standpoint of evolution, the most important difference between 1915 and 1914 work is the deliberate withdrawal of the infantry firingline behind the military crest of the position. This was far from being universal in practice, but it had become the recognized ideal. As in permanent fortification, so also in field, the preservation of the close-defence element from bombardment was the motive of abandoning the command of the foreground. Stress begins to be laid, especially, on the local flanking of wire by machine-guns in covered emplacements. But - again as in permanent fortification - the needs of the artillery observer frequently conflict with those of the close-defence; and each local problem, solved according to the value attached to these conflicting requirements at one time, has to be re-solved by fighting when the conditions change.

In this phase there are no marked innovations in the materials employed. The works are cut deeply in the earth and revetted by the conventional sandbags, hurdling, etc. of the text-books; details typical of trench-warfare fighting, such as trench blocks, sap-heads, island traverses, have been added. The " shelter " has become the " dugout," roofed in by layers of logs, house material, or in exceptional cases railway iron, and as a rule, though not always, withdrawn from the front line into one of the parallels behind. Guns are casemated, and dugouts provided for their ammunition and personnel as well as for a variety of command posts, aid posts, signal offices and the like.

In the second phase, which grew by degrees out of the first, the technique of fortification is modified owing to new conceptions of general tactics and to the bringing in of different materials. This is the phase that will remain forever typical of the western front campaigns. Its chief tactical characteristic is intense preparation and concentration of means of all sorts on the attack front, with a corresponding absence of the surprise element. The first consequences for fortification were: (a) the deepening of forward dugouts, and their construction by mining methods which leave the natural earth undisturbed; (b) the quasi-certainty of losing the first trench system, at least temporarily, with the certainty of its being reduced to a tormented ruin in any case; (c) reliance for closedefence upon the concealed machine-gun almost to the exclusion of other weapons; (d) multiplication of systems instead of multiplication of trenches in a system; (e) more elaborate provision for the maintenance of supply for forward troops, represented in practice by the accumulation of stores in forward dumps of all sorts. It was in positions organized on these lines that the Germans fought out the battle of the Somme. But their Higher Command did not at first accept the implications of (b), and paid a heavy penalty for attempting the impossible task of holding the devastated front line in strength. Especially heavy was their loss in prisoners, due to the " deep " dugout, which was so deep that the men could not get out of it in time to man the trenches. Yet without adequate protection it was impossible to hold the front line, or any other line, under bombardment, even after converting it into a line of machine-gun posts; and the first modification due to the Soinme experience was the introduction of the only alternative to the deep dugout, viz. concrete. This was a step of great importance, both tactically and technically. Concrete had already been used, here and there, to reinforce parts of an earth-system, and was constantly employed in various back-area installations. But it was not till the latter part of 1916 that it became the principal material of " field " fortification. Instructions for mixing quick-setting concrete, and even for making ferro-concrete, on the spot begin to figure in training manuals, and in the chief belligerent countries a new industry springs up for the manufacture of standard concrete elements, which are transported to the scene of operations and built up as required. The timber, mine-gallery framing hitherto used in dugouts gives place largely to standard semi-cylinders of corrugated iron, also made at home and transported to the front in great numbers. Thus the ideal dugout for troops becomes a concrete structure lined with corrugated iron, proof against all field and medium calibres without being buried to any great depth; and the machine-gun emplacement and its dugout are fused in one concrete or ferro-concrete structure of the casemate type, famous in 1917 under the name of " pillbox " or " Mebu " (in full MaschinengewehrEisenbeton Unterstand). With this, the trench itself becomes of secondary importance. Though tradition and its undoubted usef ulness in all circumstances except those of great-battle days made the command in all countries exceedingly unwilling to give up the trench altogether, it is the chain of dugouts and machine-gun casemates, which, with the wire, defines the " position " of positionwarfare at its highest development in the spring of 1917.

In the broadest sense, a " position " consists of several distinct, and more or less parallel, systems, each of which is at such a distance from the one in front that the enemy's artillery must change position - with all the attendant difficulties and delays of repairing communications through the battle-devastated area - and mount a new attack. At intervals, a switch-line, designed on the same general principles as the parallel lines, is drawn so as to bolt off the area which may be lost in one battle phase. Between the first and second systems lies a zone in which a very active infantry resistance is carried on by means of " strong points," which are no longer labyrinths of the 1915 type but well protected and wired-in machinegun strongholds, resembling in principle the traditore batteries of permanent fortification. With these as a basis, light trench-work will be run out in the directions required by the battle-situation from time to time, e.g. so as to fence off a lost sector; and for this purpose conveniently situated shell-holes are made defensible so as to constitute the framework of such lines. Tunnelling is largely used for short communications, especially where forward slopes have to be held so as to include artillery observation positions. The trench system bequeathed by the older methods is now broad (12 ft.) as well as deep, and is utilized as a covered-way, defensible in emergencies, rather than as a battle element. From 3-ft. to 5-ft. thickness of concrete (or ferro-concrete) over corrugated iron covers the casemated guns and machine-guns. With dugouts of similar design (the old deep dugouts are now sealed up or filled up by order of the command) these constitute the new defence system, whether disposed in lines or belts along the frontal systems or switch-lines, or scattered in the intermediate areas. Wire is now disposed in very wide belts, and does not conform to the direction of the trenches but lies in irregular salients and reentrants, each part under the eye of some machine-gun or trench-gun casemate. In short, the site of the " position " is governed by the needs of artillery observation, and by those of supply, while the organization of the site is based on the time, labour and material available for the construction of casemates and dugouts of concrete.  But " trench " warfare has thereby divorced itself from the element which has been its characteristic for ages, the trench, and from June 1917 to the close of the World War is the period of zonefortification. The elements of defence are now dispersed in innumerable " nests," containing machine-guns, anti-tank guns, or counterattack groups. The " position " attains hitherto unheard-of depth, and its forepart is held only by outposts, whose function is to police the ground in quiet times and give warning of assault in battle. Behind the " outpost zone " lies the " battle zone," in which machine-gun and anti-tank-gun defence break up the attack, which a counter-stroke then sweeps back to its starting points. As a precaution, a " rear zone " is organized behind the battle-zone. But field-fortification has at this stage been completely merged in field tactics, which it assists, no longer by ingenious tracing of lines of defence, clearances of foreground, economical disposition of working parties and materials, but simply by placing, at points indicated by the tactician, standard casemates and personal shelters, borrowed from the practice and made with the materials of permanent fortification.

Thus, while permanent fortification is concentrating its stock of ideas and devices upon the task of holding a long front of diminished depth, without any yielding of ground whatever, against the heaviest bombardments, field tactics have become fluid and mobile, ground being lost and won almost as readily as in the days of " man oe uvre " warfare, though by different methods. It is at the points of junction between permanent fronts and field fronts, where particular features of ground are neither quite indispensable nor yet of negligible importance, that a field-fortification of the future will presumably find its proper scope. Like the zone-fortification of 1917-8, but to a further extent, it will have at its disposal many of the forms and methods of permanent fortification. (The illustrations to this article are reproduced by permission of MM. The Librairie Berger-Levrault, of Paris, from La Revue du Genie Militaire.) (C. F. A.)