For centuries Europeans had mistakenly believed an all-water route across the North American continent existed. This “Northwest Passage” would afford the country that controlled it not only access to the interior of North America but also—more importantly—a relatively quick route to the Pacific Ocean and to trade with Asia. The Spanish, French, and British searched for years before American explorers took up the challenge of finding it. Indeed, shortly before Lewis and Clark set out on their expedition for the U.S. government, Alexander Mackenzie, an officer of the British North West Company, a fur trading outfit, had attempted to discover the route. Mackenzie made it to the Pacific and even believed (erroneously) he had discovered the headwaters of the Columbia River, but he could not find an easy water route with a minimum of difficult portages, that is, spots where boats must be carried overland.
Many Americans also dreamed of finding a Northwest Passage and opening the Pacific to American commerce and influence, including President Thomas Jefferson. In April 1803, Jefferson achieved his goal of purchasing the Louisiana Territory from France, effectively doubling the size of the United States. The purchase was made possible due to events outside the nation’s control. With the success of the Haitian Revolution, an uprising of slaves against the French, France’s Napoleon abandoned his quest to re-establish an extensive French Empire in America. As a result, he was amenable to selling off the vast Louisiana territory. President Jefferson quickly set out to learn precisely what he had bought and to assess its potential for commercial exploitation. Above all else, Jefferson wanted to exert U.S. control over the territory, an area already well known to French and British explorers. It was therefore vital for the United States to explore and map the land to pave the way for future white settlement.