The Conquest of California took place during the Mexican–American War, which was declared on May 13, 1846, between the United States and Mexico. News of Congress' declaration of war took almost three months to reach the Pacific coast. U.S. consul Thomas O. Larkin, stationed in Monterey, was concerned about the increasing possibility of war and worked to prevent bloodshed between Americans and the small Mexican military garrison at the Presidio of Monterey, which was commanded by José Castro. The campaign would ultimately determine control of Alta California, then part of Mexico.
The military campaign involved multiple forces converging on California. United States Army Captain John C. Frémont led a U.S. Army Corps of Topographical Engineers expedition with about 60 well-armed men. Frémont crossed the Sierra Nevada range in December 1845 and reached the Oregon Territory by May 1846, when he received word that war was imminent. The campaign lasted from 1846 to 1847 and involved various military engagements across Alta California as American forces moved to secure the territory.
The Conquest of California ended with the signing of the Treaty of Cahuenga by military leaders from both the Californios and Americans. This treaty marked the conclusion of active military operations in Alta California and resulted in the territory coming under United States control. The campaign represented a significant phase of the Mexican–American War and established American dominion over what is now the state of California.
The Mexican-American War (1846–1848) grew from the annexation of Texas (1845) and a disputed border between Texas and Mexico at the Rio Grande. President James K. Polk ordered US troops under General Zachary Taylor into the contested zone; after a skirmish that killed American soldiers, Congress declared war in May 1846. US forces won a series of engagements — Palo Alto, Monterrey, Buena Vista — before General Winfield Scott led an amphibious landing at Veracruz and an overland campaign to Mexico City, which fell in September 1847. The Treaty of Guadalupe Hidalgo (February 1848) transferred California, Nevada, Utah, most of Arizona and New Mexico, and parts of Colorado and Wyoming to the United States in exchange for $15 million and assumption of $3.25 million in claims — roughly 525,000 square miles, a 67 percent expansion of US territory. The war's outcome immediately reopened the slavery question: the Wilmot Proviso, debated throughout the war, proposed banning slavery from any territory acquired from Mexico, foreshadowing the sectional crisis of the 1850s.
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