In 1804, a year after the Louisiana Purchase, President Jefferson sent an expedition under Meriwether Lewis and William Clark to explore the purchase and to continue on to the Pacific Ocean. The Lewis and Clark Expedition traveled up the Missouri River, spent the winter of 1804 to 1805 with the Mandan people, and with the help of a Shoshone woman named Sacagawea traveled west along the Snake River to the Columbia River and on to the Pacific. Even as they traveled, mounted bands of Sioux were conquering the northern Great Plains. The Sioux had already cut off the Pawnee, Otoe, and other peoples of the lower Missouri from the western buffalo herds and were threatening the Mandan and other agricultural peoples on the upper reaches of the river. Throughout the first half of the 19th century, epidemics of European diseases traveled up the Missouri River. The worst of them came in the 1830s, when smallpox killed half the Native Americans along the river. The Sioux, who lived in small bands and moved constantly, were not as badly hurt as others were. They used that advantage to complete their conquest of the northern sections of Jefferson’s great “Empire of Liberty.”
Farther south, white settlers were crossing the Mississippi onto the new lands. Louisiana, already the site of New Orleans and of Spanish and French plantations, became the first state west of the Mississippi in 1812. Southerners were also moving into the Arkansas and Missouri territories. Missouri entered the Union in 1821, Arkansas in 1836. Settlers also began moving into Texas, in the northeastern reaches of the Republic of Mexico, which won its independence from Spain in 1821. Mexico at first encouraged them but demanded that new settlers become Catholics and Mexican citizens. Mexico also demanded that they respect the Mexican government’s abolition of slavery within its territory.
Settlers tended to ignore these demands, and they continued to stream into Texas even when the Mexican government tried to stop the migration. By 1835 the 30,000 Americans in Texas outnumbered Mexicans six to one. When the Mexican government tried to strengthen its authority in Texas, the American settlers (with the help of many of the Mexicans living in that province) went into an armed revolt known as the Texas Revolution. Volunteers from the southern United States crossed the border to help, and in 1836 the Americans won. They declared their land the independent Republic of Texas and asked that it be annexed to the United States. The question of Texas annexation would stir national politics for the next ten years.
Americans considered the plains that formed most of the Louisiana Purchase (the lands over which the Sioux had established control) to be a desert unsuitable for farming. Congress designated the area west of Arkansas, Missouri, and Iowa and north of Texas as Indian Territory in the 1840s. But Americans were already crossing that ground to reach more fertile territory on the Pacific, in California and Oregon (which included present-day Washington and much of present–day British Columbia).
These lands were formally owned by other countries and occupied by independent indigenous peoples. California was part of Mexico. The Oregon country was jointly occupied (and hotly contested) by Britain and the United States. American settlers, most of them from the Ohio Valley, crossed the plains and poured into Oregon and the Sacramento and San Joaquin valleys in California after 1841. As populations in those areas grew, members of the new Mormon Church, after violent troubles with their neighbors in Ohio, Missouri, and Illinois, trekked across the plains and the Rocky Mountains in 1847 and settled on Mexican territory in the Salt Lake Valley. "USA" © Emmanuel BUCHOT, Encarta, Wikipedia.
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