Word of Congress' declaration of war reached California by August 1846. American consul Thomas O. Larkin, stationed in Monterey, worked successfully during the events in that vicinity to avoid bloodshed between Americans and the Mexican military garrison commanded by General José Castro, the senior military officer in California.

Captain John C. Frémont, leading a U.S. Army topographical expedition to survey the Great Basin, entered Sacramento Valley in December 1845. Frémont's party was at Upper Klamath Lake in the Oregon Territory when it received word that war between Mexico and the U.S. was imminent; the party then returned to California.

Mexico had issued a proclamation that unnaturalized foreigners were no longer permitted to have land in California and were subject to expulsion. With rumors swirling that General Castro was massing an army against them, American settlers in the Sacramento Valley banded together to meet the threat. On June 14, 1846, 34 American settlers seized control of the undefended Mexican government outpost of Sonoma to forestall Castro's plans. One settler created the Bear Flag and raised it over Sonoma Plaza. Within a week, 70 more volunteers joined the rebels' force, which grew to nearly 300 in early July. This event, led by William B. Ide, became known as the Bear Flag Revolt.