Thomas Jefferson had long wanted to explore the West and to find a water route to the Pacific Ocean. Before the land belonged to the United States, he had asked Meriwether Lewis if he would be interested in leading an exploration expedition to the Missouri River basin with the intent of finding passage to the Pacific.
Negotiations with France for the purchase of the Louisiana Territory began early in 1803, by April an agreement was signed; and by October, this constitutionally questionable purchase was ratified by Congress. The lands purchased were bordered by the Mississippi River on the east and extended west to the Rocky Mountains and north into British North America. Virtually all of South Dakota was included in the purchase. By May 1804, Lewis and Clark were on their way up the Missouri River.
The expedition stuck with the Missouri River and didn’t head into the Black Hills , but the successful expedition to the Pacific Coast fired the imagination of the American people, affirmed American ownership of the Missouri River Basin, served as the starting point of American negotiations with the native inhabitants of the West, and opened the country to the fur trade.
The American people now confronted the same issues the Spanish had before them, specifically how to establish friendly relations with the tribes and how to stave off any further invasion by the British into American soil. Unfortunately the white relationship with the Teton Sioux was not improved upon by the Lewis and Clark Expedition. Lewis wrote in his journal:
…the vilest miscreants of the savage race, and must ever remain the pirates of the Missouri, until such measures are pursued by our government, as will make them feel a dependence on its will for their supply of merchandise.…