The Conquest of California, also known as the Conquest of Alta California or the California Campaign, was an important military campaign of the Mexican–American War carried out by the United States in Alta California (modern-day California), then a part of Mexico. The conquest lasted from 1846 into 1847, until military leaders from both the Californios and Americans signed the Treaty of Cahuenga, which ended the conflict in California.
Background
When war was declared on May 13, 1846 between the United States and Mexico, it took almost three months for definitive word of Congress' declaration of war to reach the Pacific coast. U.S. consul Thomas O. Larkin, stationed in the pueblo of Monterey, was concerned about the increasing possibility of war and worked to prevent bloodshed between the Americans and the small Mexican military garrison at the Presidio of Monterey, commanded by José Castro.
United States Army Captain John C. Frémont, on a survey expedition of the U.S. Army Corps of Topographical Engineers with about 60 well-armed men, crossed the Sierra Nevada range in December 1845. They had reached the Oregon Territory by May 1846, when Frémont received word that war between Mexico and the U.S. was imminent.
Bear Flag Revolt
On June 14, 1846, the Bear Flag Revolt occurred when some 30 rebels, mostly American pioneers, staged a revolt in response to government threats of expulsion and seized the small Mexican Sonoma Barracks garrison, in the pueblo of Sonoma north of San Francisco Bay. There they formed the California Republic, created the "Bear Flag", and raised it over Sonoma. Eleven days later, troops led by Frémont, who had acted on his own authority, arrived from Sutter's Fort to support the rebels. No government was ever organized, but the Bear Flag Revolt has become part of the state's folklore. The present-day California state flag is based on this original Bear Flag, and continues to display the words "California Republic."
Northern California
Prior to the Mexican–American War, preparations for a possible conflict led to the U.S. Pacific Squadron being extensively reinforced until it had roughly half of the ships in the United States Navy. Since it took 120 to over 200 days to sail from Atlantic ports on the east coast, around Cape Horn, to the Pacific ports in the Sandwich Islands and then the mainland west coast, these movements had to be made well in advance of any possible conflict to be effective. Initially, with no United States ports in the Pacific, the squadron's ships operated out of storeships that provided naval supplies, purchased food and obtained water from local ports of call in the Sandwich Islands and on the Pacific coast. Their orders were, upon determining "beyond a doubt" that war had been declared, to capture the ports and cities of Alta California.
Commodore John Drake Sloat, commander of the Pacific Squadron, on being informed of an outbreak of hostilities between Mexico and the United States, as well as the Bear Flag Revolt in Sonoma, ordered his naval forces to occupy ports in northern Alta California. Sloat's ships already in the Monterey harbor, the , , and , captured the Alta Californian capital city of Monterey in the "Battle of Monterey" on July 7, 1846 without firing a shot. Two days later on July 9, , which had been berthed at Sausalito, captured Yerba Buena (present-day San Francisco) in the "Battle of Yerba Buena", again without firing a shot. On July 15, Sloat transferred his command to Commodore Robert F. Stockton, a much more aggressive leader. Convincing news of a state of war between the U.S. and Mexico had previously reached Stockton. The 400 to 650 marines and bluejackets (sailors) of Stockton's Pacific Squadron were the largest U.S. ground force in California. The rest of Stockton's men were needed to man his vessels.