1844—In February, 1844, Captain John C. Frémont and party, on exploring expedition for the War Department at Washington, cross the Sierra Nevada, to Sutter's Fort, and traverse California from north to south.
1845—Negotiations for the purchase of California are resumed in 1845 by President James K. Polk. The American consul at Monterey, Thomas O. Larkin, is appointed "confidential agent" for the United States, and is instructed to keep watch against any scheming by France or Great Britain, and to influence the California people to unite themselves with the Republic.
1845—In the winter of 1845-1846 Frémont again leads a party to Sutter's Fort, and on toward the coast. He is ordered out; proceeds up for Oregon, and is recalled, May 8, 1846, into California by a naval officer with dispatches for him.
1846—June, 1846, American settlers and adventurers, in the neighborhood of Sutter's Fort, revolt against the Mexican government of California; June 14 they capture Sonoma, north of San Francisco, where they raise the Bear Flag and proclaim California to be an independent republic. Frémont aids the revolution.
1846—Following news of war between the United States and Mexico, on July 7, 1846, Commodore John D. Sloat raises the American flag over Monterey; on July 9 it is raised over San Francisco and Sonoma; on July 11, over Sutter's Fort; on August 13, Los Angeles is invested, and the flag raised there.
1847—After several engagements between the American forces and the Californians, on January 13, 1847, by the treaty of Cahuenga the Californians agree to lay down their arms.
1848—By the treaty of Guadalupe Hidalgo between the United States and Mexico, at the close of the Mexican War, and ratified at Washington, March 16, 1848, California is ceded to the United States.
1848—James Marshall, in the employ of Captain Sutter, while washing out a mill-race at Coloma, on the American River, about thirty miles west of Sutter's Fort, on January 24, 1848, discovers flakes of gold. The news spreads; it reaches Monterey, the capital, May 29, and creates intense excitement. In December the news officially reaches Washington, by communication from General Richard B. Mason and former consul James O. Larkin, and is included in President Polk's message to Congress. During 1848 $10,000,000 in gold is gathered by miners in California.
1849—In the spring of 1849 20,000 people are collected at the Missouri River, prepared to start overland 2000 miles to the California gold fields. More than 30,000 people make the land pilgrimage this year. Others sail around Cape Horn. Many others choose to cross the Isthmus of Panama, and reach the Pacific that way. The first shipload of gold seekers arrive in San Francisco February 28, 1849. San Francisco, formerly the hamlet of Yerba Buena (Good Herb), leaps from a population of 500 to one of 15,000, and the harbor has 500 vessels at anchor, flying all flags. In 1849 $40,000,000 of gold is taken from the soil by the miners.
1849—September 1, 1849, a convention to frame a State Constitution assembles at Monterey, the capital. On October 10 the constitution is adopted.