Lewis and Clark's Mission

It was Thomas Jefferson's vision that enabled the journey of Lewis and Clark. He dreamed of discovering an all-water route through the continent to the Northwest Passage, the sea route through the Arctic Ocean that connects the Atlantic and Pacific Oceans. His prime motivation was to advance trade and settlement, and he started making preparations for an expedition as early as 1792. His strongest incentive for westward expansion came in 1802, after he heard of Alexander McKenzie's overland journey across Canada. Fearing that the British might gain control of the Western territories, Jefferson sought funding for an American expedition to the Pacific -- this, in turn, led to the Louisiana Purchase [source: University of Virginia].

On April 30, 1803, Jefferson signed the Louisiana Purchase Treaty with France, undoubtedly the most important legacy of his presidency. He paid less than 3 cents an acre for the roughly 828,000-square-mile area, which doubled the size of the country [source: Encyclopedia Britannica]. The land stretched south from just above the Canadian border to New Orleans and the Gulf of Mexico, west to the Continental Divide in Colorado, and east to the Mississippi River. If it weren't for the Louisiana Purchase, the expedition of Lewis and Clark would not have been possible.

In 1803, Jefferson named his private secretary and fellow Virginian, 29-year-old Meriwether Lewis, as the commander of the Western expedition [source: University of Virginia]. Lewis (1774-1809), an accomplished sharpshooter and experienced outdoorsman, had served as an Army captain in the Northwest Territory. He chose William Clark (1770-1838), a friend from the Army, as his co-captain. Lewis felt that Clark possessed the right balance of physical strength and intellect to help lead a crew on an uncharted voyage of discovery -- and he also had a reputation as a mapmaker, so he became the official cartographer of the party. His final map of the Lewis and Clark Trail is accurate within 40 miles, though it spans a distance of 8,000 miles.

In his final instructions to the explorers, Jefferson stated that "…the object of your mission is to explore the Missouri River, and such principal streams of it, by its course and communication with the waters of the Columbia, Oregon, Colorado or any other river, may offer the most direct and practicable water communication across this continent for the purposes of commerce" [source: National Park Service].

A direct water route from sea to sea may have been Jefferson's main aim, but it wasn't his only goal for the expedition. Jefferson, like all Enlightenment thinkers, wanted to use science to shape the expedition. He told the explorers to collect, classify, document and observe the landscape, its wildlife and people with scientific precision. That's why there is such an abundance of information about the journey -- most of the members kept journals, took detailed notes and mapped the terrain as they went forward.

Manifest Destiny Manifest destiny is the belief that the United States was destined to expand westward from the Atlantic to the Pacific Ocean. The term was coined by writer John O'Sullivan in 1845. The ideology was used to justify the annexation of several territories within the United States -- and also American expansion in other parts of the world. The term didn't come into vogue until long after the Corps of Discovery completed their journey, but the group was held up as models of American expansion and dominance.

­Because the expedition crew would most likely be running into American Indian tribes on their journey, Jefferson also wanted them to be fluent in the nuances of sign language, the rituals of diplomacy and the subtle symbols of military power. The men had to study up on the social mores of different tribes -- they had to know tribal hierarchy, how to behave properly in ritual ceremonies, which tribes were dangerous and which colors were sacred or offensive.

After months of preparation, it was time to set off. Lewis gathered a partial crew in Pittsburgh, Pa.