Located on the Pacific Ocean's coast and directly close to the Mexican border, San Diego is a city in the Californian state of California, United States. San Diego is the ninth most populous city in the United States, and the second most populated city in California, according to the United States Census Bureau's 2020 population estimate of 1,386,932. (after Los Angeles). As the county capital of San Diego County, which is the fifth most populated county in the United States, with an estimated population of 3,338,330 as of 2019, the city serves as the county's administrative headquarters. There are many things to love about this city: its warm weather all year long; its natural deep-water port; its vast beaches and parks; its historic connection with the United States Navy and Marine Corps, as well as its recent rise as a healthcare and biotechnology research hub.
Although historically the home of the Kumeyaay people, the city of San Diego is sometimes referred to as the "Birthplace of California" since it was the first European settlement on what is now the West Coast of the United States, having been established in 1769. In 1542, Juan Rodriguez Cabrillo claimed the region for Spain, laying the groundwork for the colonisation of Alta California, which would take place 200 years later in the present day. Established in 1769, the Presidio and Mission San Diego de Alcalá were the first European settlements in what is now California, and together they constituted the first European colony in America. I n the year 1821 when San Diego became a member of the newly proclaimed Mexican Empire, which was then reorganised into the First Mexican Republic two years later. California became a member of the United States in 1848 as a result of the Mexican–American War, and it was admitted to the union as a state the following year.
Economic activity in San Diego is primarily fueled by the military and defense-related industries, tourism, international commerce, scientific research, and manufacturing. According to estimates from 2012, the city is the economic hub of the San Diego–Tijuana conurbation, which is the second most populous transborder metropolitan region in the Western Hemisphere (after Detroit–Windsor), with a population of about 4,922,723 people. The San Ysidro Port of Entry, which serves as the main border crossing between San Diego and Tijuana, is the busiest international land border crossing in the world, apart from those in Asia (fourth-busiest overall). San Diego International Airport, the city's main airport, is the busiest single-runway airport in the world, with over a million passengers each day.