Thomas Jefferson commissioned the Lewis and Clark Expedition in 1803 to explore the northwest territory in order to observe a transcontinental route and natural resources. In 1804, about 45 men headed by Meriwether Lewis and William Clark moved up the Missouri River, crossed the Rocky Mountains, and from the Columbia River, reached the Pacific Ocean by November 1805. They returned to St. Louis by September 1806 with great fanfare and important information on native people, plants and animals, and geography.
In 1819–20, the question of slavery's expansion into Missouri and other western territories was a matter of fierce political debate.
Jefferson and other southerners favored the "diffusion" of slaves in the west, believing that the spread of enslaved people over a larger geographic area would improve their situation and lead more swiftly to emancipation.