On January 8, 1847, about 500 Mexican militia commanded by General Jose Maria Flores hastily set up defenses on a bluff overlooking the San Gabriel River (in modern-day Montebello) for what would become known as the Battle of the San Gabriel River. This was the last of any Mexican military force to resist U.S. forces occupying Alta California. The Mexicans (or Californios, as they preferred to call themselves) faced an advancing force from San Diego of about 600 U.S. troops, marines and sailors commanded jointly by U.S. Army General Stephen Watts Kearny and U.S. Navy Commodore Robert F. Stockton. A month earlier, Flores' deputy, Andres Pico, had successfully defended an attack by Kearny's “dragoons” (mounted infantry) at San Pasqual (in modern-day San Diego County), inflicting heavy casualties on the Americans (18 killed) and forcing the Americans to retreat, badly mauled, to San Diego. Kearny himself suffered several lance wounds. This time, however, the Americans fielded a larger, better prepared and better armed force. The Californio force was mostly composed of inexperienced and poorly-equipped militia and local citizen volunteers.
The Californios opened fire on the advancing Americans in front of them as the Americans marched up to the riverbank to cross the San Gabriel River. After two hours of exchanging artillery fire and unsuccessful Californio cavalry charges (Californio cavalry, armed mostly with lances, had little chance of penetrating the withering American firepower), Flores concluded that he was unable to force the Americans to withdraw, much less defeat them. Flores ordered his forces to withdraw before they were overwhelmed or set into a panic. The Americans followed by occupying the vacated bluff, seizing abandoned Mexican artillery and setting up camp for the day.
By the following day, January 9th, many Californio volunteers, concluding the fight to be futile, simply gave up and returned to their homes. Nevertheless, even with a greatly diminished force, Flores tried in desperation to re-engage the advancing Americans at the Battle of La Mesa (in modern-day Vernon). His determined horsemen offered a more intense and more mobile fight than the events of the day before, even almost "enveloping" the American force, but the Californios were clearly outmatched and outgunned. Flores ordered a final withdrawal. Leaders from Los Angeles, seeing no further hope of keeping the Americans at bay, then offered to surrender the city peacefully.
On January 10th, the Americans occupied Los Angeles. Flores' remaining force retreated to a camp outside of Los Angeles. He knew that yet another American force, commanded by U.S. Army Lieutenant-Colonel John C. Fremont, was marching from the north to meet up with Stockton and Kearny in Los Angeles and, in fact, had arrived at the Mission San Fernando. Seeing the futility of the situation, and in light of furious threats earlier made by Commodore Stockton to shoot Flores if captured, the defeated Flores though it wise not to wait for capture. He handed what was left of his command to Andres Pico and fled for lower Mexico. Pico, with no other serious options, agreed to meet with Fremont in the Cahuenga Pass to discuss terms for surrender. Pico was also motivated by the fear that, if captured by Kearny, he too would be executed to avenge the bloody and humiliating defeat Pico’s lancers inflicted on Kearny and his men at San Pasqual. As newly-appointed Mexican Military Commander of California, Pico signed the Treaty of Cahuenga with Colonel Fremont, effectively surrendering all of Alta California to the United States.
A plaque presently marks the site of the Battle of the San Gabriel River. It is located at the northeast corner of Washington Boulevard and Bluff Road in Montebello.
Each year in early January, the Montebello Historical Society and Juan Matias Sanchez Adobe Museum has sponsored a commemoration of the Battle of the San Gabriel River with reenactors, musket fire demonstrations, displays, music and food.