The Mississippi, south to lat. 31°N, was made the western boundary. On the south the line followed the 31st parallel E to the Chattahoochee River and its junction with the Flint River, then took a straight line to the mouth of the St. Marys River, and from there to the Atlantic. The navigation of the Mississippi was to be open to the citizens of both nations.

The U.S. Territory of Missouri was set up in 1812, but settlement was slow even after the War of 1812. The coming of the steamboat increased traffic and trade on the Mississippi, and settlement progressed. Planters from the South had introduced slavery into the territory, but their plantations were restricted to a small area. However, the question of admitting the Missouri Territory as a state became a burning national issue because it involved the question of extending slavery into the territories. The dispute was resolved by the Missouri Compromise, which admitted (1821) Missouri to the Union as a slave state but excluded slavery from lands of the Louisiana Purchase north of lat. 36°30'N. By 1818, Missouri Territory had gained sufficient population to warrant its admission into the Union as a state. Its settlers came largely from the South, and it was expected that Missouri would be a slave state. In Jan., 1820, a bill to admit Maine as a state passed the House. The admission of Alabama as a slave state in 1819 had brought the slave states and free states to equal representation in the Senate, and it was seen that by pairing Maine (certain to be a free state) and Missouri, this equality would be maintained. The two bills were joined as one in the Senate, with the clause forbidding slavery in Missouri replaced by a measure prohibiting slavery in the remainder of the Louisiana Purchase north of 36°30'N lat. (the southern boundary of Missouri). The House rejected this compromise bill, but after a conference committee of members of both houses was appointed, the bills were treated separately, and in Mar., 1820, Maine was made a state and Missouri was authorized to adopt a constitution having no restrictions on slavery.

Russian fur traders had penetrated south to the California coast and established Fort Ross, north of San Francisco, in 1812. Jedediah Strong Smith and other trappers made the first U.S. overland trip to the area in 1826, but U.S. settlement did not become significant until the 1840s. In 1839, Swiss-born John Augustus Sutter arrived and established his “kingdom” of New Helvetia on a vast tract in the Sacramento valley. He did much for the overland American immigrants, who began to arrive in large numbers in 1841. Some newcomers met with tragedy, including the Donner Party, which was stranded in the Sierra Nevada after a heavy snowstorm. Political events in the territory moved swiftly in the next few years. After having briefly asserted the independence of California in 1836, the Californios drove out the last Mexican governor in 1845. Under the influence of the American explorer John C. Frémont, U.S. settlers set up (1846) a republic at Sonoma under their unique Bear Flag. The news of war between the United States and Mexico (1846–48) reached California soon afterward. On July 7, 1846, Commodore John D. Sloat captured Monterey, the capital, and claimed California for the United States. The Californios in the north worked with U.S. soldiers, but those in the south resisted U.S. martial law. In 1847, however, U.S. Gen. Stephen W. Kearny defeated the southern Californios. By the Treaty of Guadalupe Hidalgo (1848), Mexico formally ceded the territory to the United States. In 1848, the year that California became a part of the United States, another major event in the state's history occurred: While establishing a sawmill for John Sutter near Coloma, James W. Marshall discovered gold and touched off the California gold rush. The forty-niners, as the gold-rush miners were called, came in droves, spurred by the promise of fabulous riches from the Mother Lode. San Francisco rapidly became a boom city, and its bawdy, lawless coastal area, which became known as the Barbary Coast, gave rise to the vigilantes, extralegal community groups formed to suppress civil disorder. American writers such as Bret Harte and Mark Twain have recorded the local color as well as the violence and human tragedies of the roaring mini

Some topics in this essay:
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