In 1803, the US were not a big power. They had just won their independence from Britain in 1783. It was therefore a fragile independence. The expedition was meant to enhance patriotism (→ American explorers also able to do great things like France or Britain) and to insure the defence of the American territory against Canada.
Expedition was always connected to the rivalry between America and the boarder states.
Relation Lewis and Clark / Indians → to what extent did the Americans lead a more human policy towards the Indians, or was it a first step towards segregation, conquest and maybe elimination.

• First, the American continent was contested between several nations.
-- The US had just acquired independence in 1773.The western boundaries of the US (1773 – 1803) were the Mississippi River. The English empire still existed, and the Canadian fur traders were very active . They were the people that Americans feared most because of the extended British influence over the continent. The Canadian fur traders originally came from the Great Lakes region. But they had moved to the West in late 18th century because of the victory of the US over GB → Lake Athabascia
Some of these fur traders also moved to the south of Lake Winnipeg and explored the Missouri River (Mandan villages).
-- The Spanish also had power in North America → Mexico (Santa Fe : great commercial centre and upper limit of NA)
-- Gallatin thought that the Rio Grande was part of the Missouri River. The Spanish also occupied the Pacific coast up to California. Problem of Pacific coast → contested between Spain, Britain, and Russians.
-- Exploration of Captain Cook (1776 – 1779) → the island of Vancouver (important for ship trade). And so the British claimed part of Vancouver Island → Nootka Sound. When the Spanish had found out that the British had established a military camp, the Spanish shipped to the island to claim property → Nootka Sound Incident.

• Louisiana was a