Mexican American War
The United States in 1846 declared war on Mexico, and during the course of that war American military forces seized California. The war was fueled in part by feelings of Manifest Destiny, a popular sentiment in the United States that viewed the expansion of the nation as an inevitability.
The arrival in Mexican California of John C. Frémont, a loose cannon, sparked a rebellion by Anglo-Americans in the province. Their uprising became known as the Bear Flag Revolt, although the image on their banner resembled more a pig than a bear.
Military forces from the United States soon landed along the coast and marched into the interior. The Californios fought well against the Americans, scoring a victory with their long lances at San Pascual. Ultimately, however, the Mexican forces were defeated in far larger engagements elsewhere. The war ended with the signing of the Treaty of Guadalupe Hidalgo in 1848 and the cession of vast lands from Mexico to the United States. This treaty and transfer marked the end of Mexican sovereignty in California.