The Conquest of California was a military campaign during the Mexican–American War conducted by the United States in Alta California from 1846 to 1847. War had been declared between the United States and Mexico on May 13, 1846, but news of this declaration took almost three months to reach the Pacific coast. Prior to the formal outbreak of hostilities, U.S. consul Thomas O. Larkin, stationed in Monterey, worked to prevent bloodshed between American forces and the small Mexican military garrison at the Presidio of Monterey, which was commanded by José Castro. Additionally, United States Army Captain John C. Frémont led a U.S. Army Corps of Topographical Engineers expedition with approximately 60 well-armed men across the Sierra Nevada range in December 1845, reaching the Oregon Territory by May 1846 when he received word that war was imminent.
The campaign included the Bear Flag Revolt, which began on June 14, 1846. This represented a significant moment in the broader conquest of Alta California by American forces during the war.
The military campaign concluded with the signing of the Treaty of Cahuenga, which was negotiated between military leaders representing both the Californios and the Americans. This treaty formally ended the conquest of Alta California and represented the resolution of American military objectives in the region during the Mexican–American War.
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.
None in occupation
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