The Lewis and Clark Expedition from August 31, 1803, to September 25, 1806, also known as the Corps of Discovery Expedition, was the United States expedition to cross the newly acquired western portion of the country after the Louisiana Purchase. The Corps of Discovery was a select group of U.S. Army and civilian volunteers under the command of Captain Meriwether Lewis and his close friend Second Lieutenant William Clark. The expedition made its way westward, and crossed the Continental Divide of the Americas before reaching the Pacific Coast.

President Thomas Jefferson commissioned the 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 European powers attempted to establish claims in the region. The campaign's secondary objectives were scientific and economic: to study the area's plants, animal life, and geography, and to establish trade with local Native American tribes. The expedition returned to St. Louis to report its findings to Jefferson, with maps, sketches, and journals in hand.