Meriwether Lewis was born on 18 August 1774 and joined the United States Army in 1795 at the age of 20. By 1800 he had risen to the rank of captain, but his service in the army ended in 1801 when he was appointed as Secretary to the President by President Thomas Jefferson. In this position, Lewis resided in the presidential mansion, and frequently was in the company of various prominent figures in politics, the arts, and other circles. When Jefferson began to plan for an expedition across the continent, he chose Lewis as its leader. Meriwether Lewis recruited William Clark, a former commanding officer, to share command of the expedition. Their mission was to explore the territory of the Louisiana Purchase, establish trade with, and sovereignty over, the natives near the Missouri River, and claim the Pacific Northwest and Oregon Country for the United States. They also collected scientific data, and information on indigenous nations.