[203][m] Historians have differed in their assessments regarding the constitutional implications of the sale,[205] but they typically hail the Louisiana acquisition as a major accomplishment. Frederick Jackson Turner called the purchase the most formative event in American history.[198]

Lewis and Clark Expedition (1803–1806)
Main article: Lewis and Clark Expedition
Lewis and Clark on the Lower Columbia, by Charles Marion Russell, 1905
Jefferson anticipated further westward settlements due to the Louisiana Purchase and arranged for the exploration and mapping of the uncharted territory. He sought to establish a U.S. claim ahead of competing European interests and to find the rumored Northwest Passage.[206] Jefferson and others were influenced by exploration accounts of Le Page du Pratz in Louisiana (1763) and Captain James Cook in the Pacific (1784),[207] and they persuaded Congress in 1804 to fund an expedition to explore and map the newly acquired territory to the Pacific Ocean.[208]
Jefferson appointed Meriwether Lewis and William Clark to be leaders of the Corps of Discovery (1803–1806).