The USS Cyane played a significant role in the United States Navy's operations during the Mexican-American War, particularly in the Pacific Theater. The ship had been assigned to the Pacific Squadron and was actively involved in operations along the California coast during the conflict.
On 7 July 1846, Captain William Mervine, commanding officer of the Cyane, led a detachment of Marines and sailors from Commodore John D. Sloat's squadron ashore at Monterey, California. This force hoisted the American flag at the Customs House and claimed possession of the city and all of present-day California. Subsequently, on 26 July 1846, Lieutenant Colonel John C. Frémont's California Battalion boarded the Cyane, which was now under the command of Commander Samuel Francis Du Pont. The ship then sailed for San Diego, California on 29 July 1846, continuing its mission to secure American control of the California coast.
Upon arrival at San Diego, the Cyane landed Marines at La Playa, a location near San Diego. These Marines were warmly welcomed by the largely pro-American civilian population in the area. This reception reflected significant local support for American forces in California during the Mexican-American War, facilitating the relatively peaceful assertion of American authority in the region.
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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