John Charles Fremont, son-in-law of Senator Benton of Missouri, was a leader in the effort to annex California from Mexico. His first expedition to the area was in 1843, visiting Sutter at his fort, and crossing the Sierras in 1844. Most of his activities, as a matter of fact, were in Northern California.
However, during the Mexican-American War he took his battalion to San Diego where he raised the American flag over San Diego. He then march northward, and along with Commodore Robert Stockton with sailors and marines, entered Los Angeles without resistance. There followed, however, a series of defeats for the American troops when Fremont marched northward, leaving a subordinate in command.
Fremont came back to Los Angeles, arriving in the vicinity of Cahuenga Pass on January 12, 1847. After a day of negotiation, Andres Pico surrendered in the Treaty of Cahuenga at Campo de Cahuenga on January 13th. Fremont was named military governor of California. but refused to obey orders from General Stephen Kearny. Fremont was removed from office, court-martialed, found guilty, and resigned from the army.
He occupied the San Fernando Mission for a time during the War.