In 1846, during the Mexican-American War that was triggered by a border dispute in Texas, the 7th New York Volunteer Infantry Regiment occupied the crumbling adobes at the Presidio. The U.S. Regular Army took over the post the following year.

This large military reservation at the Golden Gate developed into the most important Army post on the Pacific Coast. Over time its armaments evolved from smooth bore cannons to modern missiles. It became the nerve center of a coastal defense system that eventually included Alcatraz and Angel Island and that reached as far north as the Marin Headlands and as far south as Fort Funston (all these former military lands were later incorporated into the Golden Gate National Recreation Area). Eventually, there were five distinct posts at the Presidio, each with its own commander: the Main Post, Fort Point, Letterman Hospital, Fort Winfield Scott, and Crissy Army Air Field. Also on the 1,491-acre reservation were a Coast Guard lifesaving station and a U.S. Public Health Service Hospital.

The American Presidio looked in two directions. From 1847 to about 1890, the Presidio defended San Francisco and also participated in the Indian Wars in the West. From the Spanish-American War of 1898 and the conquest of the Philippines to the end of the Vietnam War in 1973, the Presidio was a key link in the projection of American military power into the Pacific Basin and further west onto the mainland of Asia.

At the close of the nineteenth century, the Army began extensive forestation of the Presidio, turning grassy hills into dark forests that acted as windbreaks and also beautified the post. In 1874, the Presidio became an "open post" with civilian access, except during wartime. In 1888, Hittell's Guide Book to San Francisco noted that "General McDowell, late commanding the Pacific military division of the national army, made fine roads through the Presidio reservation, planted trees, and commenced other improvements, so as to convert it into a public park, which, in time, may rival the Golden Gate Park in its attractions." New concrete fortifications built after the 1890s indirectly preserved native plant communities on the dramatic Pacific bluffs by making them off-limits.

The Presidio offers a window into the changes in American society over a span of almost 150 years. The shift from an originally bachelor society where only officers could marry to a community with families and children, advances in modern medicine and health care at Letterman Hospital, the introduction and expansion of the role of women in the military, the racial integration of the armed forces in advance of American society at large, all are integral to the Presidio's history and reflect American national history. The Presidio is home to one of the nation's finest collections of fortifications, landscapes, buildings, structures and artifacts related to military history. It also embraces the oldest national cemetery on the West Coast, the final resting place of many Medal of Honor awardees.

Read about the Post to Park transition.