Ex post facto laws

An ex post facto law is a law that makes illegal an act that was legal when committed, increases the penalties for an infraction after it has been committed, or changes the rules of evidence to make conviction easier. The American Constitution prohibits the making of ex post facto law by either Congress or the states.

An important historical invocation of an ex-post-facto defense can be seen in the Nuremberg trials, where some defendants argued that many of the crimes that they were charged with did not exist until they had been drafted by the Allies.