What was the first United States expedition to cross the Louisiana Purchase called?

The first United States expedition to cross the Louisiana Purchase was the Lewis and Clark Expedition.

Also called the Corps of Discovery Expedition, it began in 1804 after President Thomas Jefferson commissioned Meriwether Lewis and William Clark to explore the newly acquired territory. The expedition departed from Camp Dubois near present-day Hartford, Illinois, on May 14, 1804, and reached the Pacific Ocean in 1805.

The explorers mapped rivers, recorded plants and animals, studied geography, and established diplomatic contacts with Native nations. Sacagawea traveled with the party and helped communicate with several communities; her husband, Toussaint Charbonneau, also joined the expedition. The group returned to St. Louis on September 23, 1806, after traveling for 862 days.

The expedition did not “discover” an unknown wilderness: Indigenous peoples had long lived throughout the region, and European explorers had previously visited portions of it. Its importance lies in the information it gathered for the United States and the government’s effort to assert control over the new territory.