In June 1846, thirty-three American immigrants in Alta California who had entered the Mexican department without official permission rebelled against the Mexican government. Their grievances centered on being denied the right to buy or rent land and facing threats of expulsion. Mexican officials had grown increasingly concerned about a coming war with the United States and the rising influx of American settlers into California, creating tensions that precipitated the uprising. The rebellion was covertly encouraged by U.S. Army Brevet Captain John C. Frémont, and it occurred amid the recent outbreak of the Mexican–American War, adding to Mexico's mounting difficulties during this period.
The rebels, numbering thirty-three American immigrants, took control of an area north of San Francisco, in and around what is now Sonoma County in California. The insurgents elected military officers to lead their movement, though no formal civil governmental structure was ever established during the brief republic's existence. The rebels raised a flag in Sonoma that bore the name "California Republic" and featured a silhouette of a California grizzly bear, which came to be known as the Bear Flag. This flag symbolized their aspiration to form a republican government under their own control, independent of Mexican authority.
The California Republic existed for only twenty-five days, from June 14 to July 9, 1846, making it a short-lived unrecognized breakaway state from Mexico. Though the rebels succeeded in militarily controlling their designated territory around Sonoma County during this period, the republic's brief existence and lack of international recognition meant it had limited lasting political impact. The rebellion contributed to the broader upheaval in California during the Mexican–American War and demonstrated the growing American presence and assertiveness in the Mexican territory, foreshadowing the eventual American acquisition 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.
None at Sonoma — Vallejo surrendered
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