The "Siege of Los Angeles" was a military occupation by the United States Marines of the Pueblo de Los Angeles during the Mexican-American war.
On August 13, 1846, early in the American invasion of California during the Mexican-American War, the US Navy, under Commodore Robert F. Stockton, sent an occupying force of fifty US Marines, under USMC Captain Archibald H. Gillespie into the Pueblo de Los Angeles and raised the American flag without opposition, as Mexican government officials fled Alta California. A rudimentary US barricade called Fort Hill was hastily built overlooking the small pueblo.
The martial law imposed on the surprised and confused pueblo citizens by Captain Gillespie soon ignited a popular uprising. The Californios, organized under José Mariá Flores, a Mexican Officer who remained in California, Jose Antonio Carrillo and Andres Pico, and assembled a vaquero Lancer force that began the fight to break the "Siege of Los Angeles" on September 22, 1846.
Gillespie's marines were able to resist an initial attack on the government house in town and regrouped on Fort Hill, where they strengthened their fortification with sandbags and mounted a cannon. Within a short time, the Californio force grew to just over sixty men, with several Californio citizens voicing strong opposition to the American invaders. Flores offered an ultimatum: leave within twenty-four hours or face attack. Gillespie, remembering the Americans at the Alamo, chose to withdraw from Los Angeles, and on September 30, 1846, American forces retreated to their Brig Savannah, berthed in San Pedro Bay, with Stockton's fleet. This was the beginning of the unanticipated Californio resistance to the American invasion of Southern California.