The Capture of Yerba Buena occurred during the Mexican–American War as part of a broader U.S. strategy to secure California. Six months before the United States declared war on Mexico on May 13, 1846, President James Polk had ordered the U.S. Navy's Pacific Squadron to occupy every important port and city in California, using force if necessary once war was confirmed. The engagement was precipitated by concerns from U.S. Consul Thomas O. Larkin in Monterey, who, worried about the increasing possibility of war, requested that Commodore John D. Sloat of the Pacific Squadron send a warship to protect American citizens and interests in Alta California.
The USS Portsmouth, commanded by John Berrien Montgomery, arrived at Monterey on April 22, 1846, in response to Larkin's request. The ship moved to San Francisco Bay by mid-May and anchored at Sausalito. While stationed there, a messenger from American Captain John C. Frémont's expedition requested supplies from the Portsmouth. After the ship returned to Monterey, Montgomery conveyed information about Frémont's presence in California to Larkin, and the two men decided to coordinate further action regarding the occupation of Yerba Buena.
The capture of Yerba Buena, now San Francisco, was accomplished by the U.S. Navy without firing a shot, representing a bloodless occupation of this strategically important California port. This engagement exemplified the broader Mexican–American War campaign to secure California's key coastal positions for the United States, establishing American control over the settlement that would become one of the nation's most significant cities.
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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