Six months before the United States declared war on Mexico on May 13, 1846, President James Polk issued orders to 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 at Yerba Buena occurred within this strategic context, as U.S. officials in California grew increasingly concerned about potential conflict. U.S. Consul Thomas O. Larkin in Monterey, anxious about rising tensions, requested that Commodore John D. Sloat of the Pacific Squadron send a warship to protect American citizens and interests in Alta California. In response, the USS Portsmouth, commanded by John Berrien Montgomery, arrived at Monterey on April 22, 1846, and proceeded to San Francisco Bay by mid-May, anchoring at Sausalito. The ship's presence was further motivated by a messenger from American Captain John C. Frémont's expedition who requested supplies, demonstrating the interconnected nature of American military and civilian operations in the region during this period of escalating tensions.
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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