The invasion of western Mexico
An American army from Fort Leavenworth, commanded by Stephen Kearny, occupied Santa Fe in August 1846, declared New Mexico a territory of the United States, and established a provisional government, without a shot being fired. Kearny proclaimed himself its military governor. The army then moved west to California, leaving behind a garrison under Sterling Price, with Charles Bent as governor of New Mexico. In January, 1847, an insurgency arose against the Americans which severely wounded Bent, including scalping him in his home in the presence of his family. A series of battles between the American garrison and the insurgents quashed the rebellion.
American settlers in California rebelled against the weak Mexican government there, and ships of the US Navy seized Yerba Buena Bay (now San Francisco) supporting the settlers and American troops under John Fremont. Meanwhile Kearny’s army marched overland to Southern California. Bloody fighting took place between Kearny’s troops and Mexican forces, but the Mexicans were steadily pushed back, and by winter the American’s occupied Los Angeles and most of the other settlements in California. In January Mexican commanders agreed to surrender, and in the Treaty of Cahuenga ceded all of California north of the Baja Peninsula to the United States.
In the spring of 1847 the United States Pacific Squadron destroyed the Spanish shipping in Baja California, and troops and Marines raided its major ports and facilities. Mexican forces undertook campaigns to reclaim the ports, and a series of battles occurred throughout 1847, but the Americans retained the ports until evacuating them following the cessation of hostilities. In Baja California, the United States had support of Mexican citizens unhappy with the national government since Spain was ousted, and many of those citizens accompanied the Americans when Baja California was evacuated at the end of the war.
The seizure of what was then known as Upper California (Alta California) was the true goal of the Polk Administration in its pursuit of war with Mexico. The region known as Nueva Mexico, between the state of Texas and the Sierras, was of little concern other than it provided a contiguous link to the west coast settlements. California, with its fertile valleys and fine harbors, was desirous both to curtail European settlement on the continent through the expansion of Canada and Mexico, and to provide the United States with unimpeded access to the Pacific. Baja California was less desirable, and in the treaty which ended the war was returned to Mexico, despite American occupation.
Shortly after the end of the war gold was discovered in California, and the sparsely settled region was inundated with those hoping to strike it rich. California’s population exploded. The region, rich with Spanish names and traditions, became home to great ports and the former settlement of Yerba Buena grew quickly into the city of San Francisco. The seizure of California and Nueva Mexico diminished the acreage of Mexico by nearly half, and ensured that the United States would occupy the North American continent from the Canadian border to as far south as it demanded in treaty negotiations with the Mexicans.