President Thomas Jefferson commissioned The Lewis and Clark Expedition shortly after the Louisiana Purchase in 1803 to explore and to map the newly acquired territory, to find a practical route across the western half of the continent, and to establish an American presence in this territory before Britain and other European powers tried to claim it. Also known as the Corps of Discovery Expedition, it was the first American expedition to cross the western portion of the United States, and passed through the Continental Divide of the Americas to reach the Pacific coast. The Corps of Discovery was a selected group of US Army volunteers under the command of Captain Meriwether Lewis and his close friend Second Lieutenant William Clark. The Expedition departed from Camp Dubois, now known as Wood River, Illinois on May 14, 1804 and returned to St. Louis, Missouri on September 23, 1806.