Early California: pre-1769–1840s: Mexican War

The ideology of Manifest Destiny was a prime motivation for the acquisition of California in the 1840s. This philosophy linked the expansion of U.S. territory with extending the "boundaries of freedom". The belief that Anglo-Saxon social and political systems were the height of human achievement led some to the conclusion that these systems would one day extend across the continent. However, non-whites were seen as incapable of practicing this "freedom". Poet and author Walt Whitman expressed this view when he stated, "What has miserable, inefficient Mexico--with her superstition, her burlesque upon freedom, her actual tyranny by the few over the many--what has she to do with the great mission of peopling the new world with a noble race? Be it ours to achieve that mission!"

To accomplish this mission, a force of 300 mounted soldiers was organized at Fort Leavenworth in Kansas, under the command of Col. Stephen W. Kearney, in the summer of 1846. This "Army of the West" was sent to California to seize Monterey and San Francisco. By early December, the Americans were about 40 miles east of San Diego. Here, Kearney learned that an equal force of Californios was encamped a few miles away in San Pasqual Valley. Rather than march around this force, Kearney decided to attack.

In the early morning hours of Dec. 5, 1846, 100 U.S. soldiers moved into San Pasqual Valley. Already alerted to their presence, the Californios fled. Spurred on by this apparent retreat the Americans began a disorganized pursuit. As the Americans raced along the length of the valley, the Californios suddenly turned and charged straight into them. In 15 minutes of violent combat, 21 U.S. soldiers were killed and 17 more were wounded while the Californios suffered only one casualty. Having stopped the American advance, the Californios abandoned the valley. Only after the arrival of U.S. reinforcements from San Diego was Kearney's force able to make it to safety.

In the months following the "victory" at San Pasqual, the U.S. military defeated the Californios in a series of small engagements in southern California, finally forcing their surrender near Los Angeles on Jan. 13, 1847. The transfer of American rule to California following the end of the Mexican War was a severe blow to the Californios. Forced to prove legal ownership of their land under U.S. law, many lost their homes. As the basis of the Mexican socioeconomic system had been land ownership, this loss forced many Californios into menial labor and poverty. The mass migration of white miners brought about by the gold rush of 1849 soon made California-born Mexicans a minority in their own homeland. Likewise a series of laws, such as the anti-vagrancy act of 1855 and an act negating the requirement that laws in California be translated into Spanish, further disconnected California from its Hispanic heritage. This would continue into the 20th century, as Mexican migrants faced discrimination and social inequality when they came to California looking for a new start.