On January 8, 1847, about 500 Mexican militia commanded by the young General José María Flores (only about 28 years old, but appointed general because he was only remaining professional military officer) 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 significant Mexican military force able to offer serious resistance to 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, composed of marines, sailors, army dragoons and California volunteers commanded by U.S. Navy Commodore Robert F. Stockton and, slightly secondarily, by U.S. Army General Stephen Watts Kearny. 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 serious 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 entrenched Californios opened fire on the advancing Americans in front of them in the open 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 withering American firepower), Flores concluded that he could not force the Americans to withdraw, much less defeat them. He ordered his forces to withdraw before they could be 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, unfortunately for the Mexican cause, many Californio volunteers, concluding the fight was futile, simply gave up and returned to their homes. Flores could do little to stop them. Nevertheless, even with a now hugely diminished force, Flores tried a desperate re-engagement with 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 day before, even almost completely "enveloping" the American force, but the Californios did not have the numbers or firepower to break through the American defensive lines. Seeing that he had nothing left with which to stop the Americans, Flores ordered a final withdrawal. Civilian 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. Dismissing suggestions of partisan resistance and seeing the complete futility of the situation (and in light of furious threats earlier made by Commodore Stockton to shoot Flores if captured), the defeated Flores thought it wise not to wait to be captured. He handed what little was left of his command to the one last officer he could trust, Andres Pico, and fled for lower Mexico. Pico, also unable to see any 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 his lancers earlier inflicted on Kearny and his men at San Pasqual. As newly-appointed Mexican Military Commander of California and the only significant Mexican authority left in California, Pico signed the Capitulation of Cahuenga with Colonel Fremont, effectively surrendering all of Mexican province 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.