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 Mexican military force to resist U.S. forces occupying Alta California. The Mexicans faced an advance from San Diego of about 600 U.S. troops commanded by U.S. Army General Stephen Watts Kearny and U.S. Navy Captain Robert F. Stockton. A month earlier, Flores' deputy, Andres Pico, successfully led an attack on Kearny's force at San Pasqual, inflicting heavy casualties on the Americans (18 killed) and forcing them to retreat to San Diego. Kearny himself suffered several lance wounds. This time, however, the Americans, fielding a more disciplined force and superior firepower, were better prepared. The Mexican defenders were composed of inexperienced and poorly-equipped militia and local citizens.
The Mexicans opened fire upon the advancing U.S. troops as they arrived at the bank of the San Gabriel River. After two hours of artillery duels and several unsuccessful Mexican cavalry charges, Flores decided that he could not force the Americans to withdraw and conceded the battle by withdrawing himself. The Americans occupied the previously defended bluff, seizing abandoned Mexican artillery and setting up camp for the day.
The following day, on January 9th, the Mexicans again engaged the advancing American force at the Battle of La Mesa (in modern-day Vernon). Although the Mexicans offered a more intense fight and managed to almost "envelope" the American force, Flores conceded that he could not stop this advance and again ordered a withdrawal. Hearing of this outcome, leaders from Los Angeles gave up hope of a successful defense and came out to surrender the city peacefully to the American commanders.
On January 10th, the Americans occupied Los Angeles. Flores' remaining force retreated to a camp in what is now Pasadena. Flores learned 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. Seeing the futility of the situation, Flores decided to turn over command to Andres Pico and flee to unoccupied Mexico. Pico, with no other serious options, agreed to meet with Colonel Fremont in the Cahuenga Pass to discuss terms for surrender. Pico was also motivated by the fear that, if he were captured by Kearny, he would be executed out of revenge for the San Pasqual battle. As newly-appointed Mexican Military Commander of California, Pico signed the Treaty of Cahuenga with Colonel Fremont, effectively surrendering all of Alto 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.