In July 1846 during the Mexican-American War, Los Angeles was the largest Mexican town in California. It was captured by a 400 man strong army in that month. However, due to the dictatorship-like rule of the garrison leader, Lieutenant Archibald Gillespie, he and his 50 man strong group were attacked by a guerrilla force of 300 led by Servuilo Varela and Leonardo Cota in September and summarily kicked out on October 4th. This gave the guerrillas control of all of Southern California for a short time.
On October 8th, Captain William Mervince landed at San Pedro and was confronted the Mexicans at the Domínguez ranch in the Battle of the Old Woman's Gun* where Mervince was chased back to the sea.
Two months later on December 6th, the Battle of San Pascual was fought near present-day Escondido where the Southern Californians confronted an army led by General Stephen W. Kerny which had just arrived from New Mexico. The short battle was won by the Americans via the decisive use of artillery.
The last battle of the Alta California campaign took place January 8-9 in the Battle of the San Gabriel River near present-day Montebello, and 4 days later at Cahuenga, the Alta California governor Pío Píco surrended to Lieutenant Colonel John C. Frémont.
* Origin of the name "Battle of the Old Woman's Gun." It was named after an old brass four-pounder cannon used in cermonial salutes. It was excavated from its burial place, the garden of Inocencia Reyes and used in the battle. (Bear Flag Rising; p. 200)
Dale L. Walker, Bear Flag Rising: The Conquest of California, 1846
Lenoard & Dale Pitt, Los Angeles A to Z