1821—By revolt of Mexico against Spain, in 1821 California becomes a Mexican province.
1826—In 1826 arrive the first Americans by land, being a party of trappers led from Salt Lake by Jedediah S. Smith.
1832—Captain Benjamin Morrell, Jr., of the American vessel Tartar, after having stopped at California publishes, in 1832, a book upon his travels, in which he urges the acquisition of California by the United States.
1835—President Andrew Jackson authorizes Colonel Anthony Butler, American official in Mexico, to purchase, if possible, for the United States, "the whole bay of San Francisco." The plan fails.
1839—July 3, 1839, arrives at Monterey Captain John August Sutter, a Swiss-American. In August he takes up a tract of land on the south bank of the American River, east from present Sacramento, and there establishes a trading post which he names New Helvetia, but which became better known as Sutter's Fort. The post grows to be a rallying place for American trappers and settlers.
1841—In November, 1841, arrive the first company of American immigrants, led by J. Bartleson and John Bidwell, from the Missouri River, along the Oregon Trail to the Salt Lake cut-off, thence down the Humboldt River and across the Sierra Nevada mountains and down the Stanislaus River. Numbering thirty-nine, they reach the ranch of Dr. John Marsh, early American settler, back of the present city of Oakland, opposite San Francisco.
1841—In October and November, 1841, the Bay of San Francisco, and the Sacramento and San Joaquin Rivers are surveyed by the Government exploring expedition under command of Captain Charles W. Wilkes, United States Navy.
1842—The Honorable Waddy Thompson, United States minister to Mexico, informs President John Tyler, April 29, 1842, that Mexico is willing to sell Texas and Upper California. He emphasizes the importance of California.
1842—October 20, 1842, Commodore Thomas ap Catesby Jones of the United States Navy raises the American flag over Monterey, thinking that war with Mexico had been declared. The next day he apologizes; but the sale of California is interrupted.
1842-43-44—The American immigration overland gradually increases in 1842, 1843, 1844, and alarms the Mexican authorities, who fear the spread of American influence. The majority of the settlers locate in Northern California.