Conquest of California

The Conquest of California, also known as the Conquest of Alta California or the California Campaign, was a military campaign during the Mexican–American War carried out by the United States in Alta California (modern-day California), then part of Mexico, lasting from 1846 to 1847, and ending with signing of the Treaty of Cahuenga by military leaders from both the Californios and Americans.

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 U.S. Army Corps of Topographical Engineers expedition with about 60 well-armed men, crossed the Sierra Nevada range in December 1845. They reached the Oregon Territory by May 1846, when Frémont received word that war was imminent.

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."

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 USS Savannah, USS Cyane, and USS Levant, captured the Alta Californian capital city in the Battle of Monterey on July 7, 1846, without firing a shot. Two days later on July 9, USS Portsmouth, 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 29, Sloat transferred his command to Commodore Robert F. Stockton, a more aggressive leader. 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 troops were needed to man his vessels.

To supplement this remaining force, Commodore Stockton ordered Captain John C. Frémont, on the U.S. Army Corps of Topographical Engineers survey, to secure 100 volunteers in addition to the California Battalion he had organized earlier. He received 160, exceeding his order. The volunteers were to act primarily as occupation forces to free up Stockton's marines and sailors. The core of the California Battalion was the approximately 30 army personnel and 30 scouts, guards, ex-fur trappers, Indians, geographers, topographers and cartographers in Frémont's exploration force, which was joined by about 150 Bear Flaggers. The American marines, sailors, and militia easily took over the cities and ports of northern California; within days they controlled Monterey, San Francisco, Sonoma, Sutter's Fort, New Helvetia, and other small pueblos in northern Alta California. Nearly all were occupied without a shot being fired. Some of the southern pueblos and ports were also rapidly occupied, with almost no bloodshed.

Prior to the U.S. occupation, the population of Spanish and Mexican people in Alta California was approximately 1500 men and 6500 women and children, who were known as Californios. Many lived in or near the small Pueblo of Los Angeles (present-day Los Angeles). Many other Californios lived on the 455 ranchos of Alta California, which contained slightly more than 8,600,000 acres (35,000 km²), nearly all bestowed by the Spanish and then Mexican governors with an average of about 18,900 acres (76 km²) each.^{[citation needed]}