The legendary journey that for the first mapped out the North American continent width-wise had its origins at the turn of the 19th century, when the U.S. population went north of five million and dreams of westward expansion grew proportionally. There was still talk of finding the Northwest Passage, the mythical water route across the continent to the distant Pacific ocean when with the stroke of a pen President Thomas Jefferson completed the Louisiana Purchase from France and doubled the nation’s size. Presently, what was needed was a survey of the new lands, and what lay beyond them, by the Pacific.

On this day, May 21, in 1804, just days after Jefferson drafted his personal secretary Captain Meriwether Lewis, Lewis drafted his comrade-in-arms William Clark to begin the expedition in search of the Northwest Passage.

Clark and Lewis made many geological, astronomical and biological observations during their trip, greatly enhancing the scientific culture of the land, but science was only an afterthought behind discovery of the Passage and the staking of claims to lands beyond. By mapping out the lands over which they passed, and putting down medals and flags, the expedition could lawfully claim them for America under the Doctrine of Discovery.