A lot happened during the ultimately successful annexation of California to the USA. Ultimately, the commander were John Charles Fremont for the USA and General Andres Pico for "Mexico.
In 1846, the Mexican territory of California was thinly populated, with small and scattered settlements of both Spanish-speaking Californios and Hispanos and English-speaking immigrants, and both were outnumbered by the Native American populations.
On June 14, 1846, 30 American settlers in Sonoma, after a night of drinking, arrested and imprisoned Lieutenant Colonel Mariano Guadalupe Vallejo and proclaimed an independent republic of California; this proved to be a short-lived "republic" and its influence never reached further than Sonoma and scattered parts of northern California. Captain John C. Frémont of the U.S. Army arrived with his soldiers at the so-called "Bear Flag Revolt" in Sonoma on June 25 and organized the rebels into a motley group calling itself the California Battalion.
On July 7, on California's Pacific coast, Sloat claimed Monterey (not to be confused with Monterrey, Nuevo León), taking formal control of California under the U.S. flag. He transferred his command to Commodore Robert F. Stockton on July 15.
On August 13, 1846, U.S. naval forces sailed into Los Angeles and raised the U.S. flag without opposition. A garrison of 50 Marines established what would become Fort Moore. However, the heavy-handed martial law of Captain Archibald Gillespie, who was acting commander of Los Angeles, ignited a popular uprising led by José Mariá Flores. Gillespie's small but oppressive garrison was thrown out on September 23 and his life was spared in a truce on the condition that he immediately leave California.
Stockton was informed of this revolt by the ¨Paul Revere¨ of California, Lean John, and promised to make quick work of the uprising and their leaders by sending Captain William Mervine and a ship to San Pedro. As Captain Mervine landed his 350 men on October 7, 1846, Gillespie, seeing Mervine and his force land, immediately scrapped the truce made with the Californios. The new expedition quickly set out for Los Angeles, anxious to cover themselves in martial glory. In a skirmish known as ¨The Battle of the Old Woman's Gun¨, mounted Californios led by Jose Antonio Carillo met Mervine's sailors, Marines, and Bear Flaggers with fire from a single cannon which took a terrible toll on Mervine's men, forcing him to retreat with his marines back to his ship Savannah, where the Californios couldn't reach them. This battle is also known as the Battle of Dominguez Rancho. During the skirmish, ten Americans were wounded, of which four later died (Bauer, 185). The Californios suffered no dead and 5 wounded. As a great number of U.S. reinforcements approached, the Californios retreated as night fell. Commodore Stockton landed in San Diego and later relieved Mervine and Gillespie with large reinforcements.
Meanwhile, General Kearny and the Army of the West (some 1,700 U.S. army troops) marched to Santa Fe, New Mexico and took control. Kearny then proceeded onward with a considerably smaller detachment of 300 dragoons along the Gila River valley, and across the deserts to California, arriving there with fewer than 150 men. General Kearny had been ill-advised by a number of U.S. soldiers, including his famous scout Kit Carson, that the Californios were cowards and would sooner run than fight. Kearny received the news that Andres Pico and his insurgents were in the vicinity. Before dawn on December 6, 1846, at San Pascual (near present day Escondido, California), General Kearny and the Army of the West, augmented by Gillespie's men, fought a pitched battle with less than 150 Californios. The Californios, renowned for their horsemanship, easily outmaneuvered the U.S. soldiers with their lassos.