How SF and California Became Part of U.S.

California became a State as a result of the Mexican American War, 1846-1848. Americans had been moving into the Mexican territory of California, called Alta California, for several years and had established a ship landing and unloading operation inside San Francisco Bay at a settlement called Yerba Buena. Although there were tensions between both governments over the new territory, Yerba Buena operated under Mexican law.
On July 9, 1846, the American warship Portsmouth landed on the San Francisco shoreline and marched soldiers to the town square. Navy Captain John Berrien Montgomery and US Marine Second Lieutenant Henry Bulls Watson raised the American flag, ordered a twenty-one gun salute, and declared the land to be part of the United States.