Thomas Jefferson, author of the Declaration of Independence and the third president of the United States, had long been interested in exploring the American West. He hoped that a water route, a "Northwest Passage," from the Atlantic to the Pacific oceans could be found. In 1792, an American sailor discovered a major river (the Columbia) in the Pacific Northwest that flowed from the Rocky Mountains to the Pacific, thus increasing the possibility that a water route across the continent might be found. If such a route existed, it would be invaluable for trade and would greatly ease travel, thereby increasing settlement opportunities for the American people.

When Jefferson took office in 1801, the westernmost boundary of the United States was the Mississippi River. Beyond that lay a large tract of land, claimed by France, known as the Louisiana Territory. Beyond the Louisiana Territory lay two other, even less explored territories claimed by Spain, Russia, Great Britain, and the United States.

Little was known about these lands. Jefferson had heard many tales about what might be out there, including active volcanoes (this claim was correct), a mountain made out of salt, living woolly mammoths, and giant ground sloths. He knew that many Native American tribes made their homes in the West, and he wished to learn as much as possible about who they were and how they lived.

In 1803, Jefferson asked Congress, in a secret letter, to authorize an expedition to explore the West. Congress approved Jefferson's request, and Jefferson began planning the expedition. Later that same year, the United States purchased the vast Louisiana Territory from France, which more than doubled the size of the U.S. This made exploration even more necessary, particularly because the boundaries of the Louisiana Territory were not very specific, and Jefferson saw an opportunity to extend the borders of the United States as far westward as possible.