Thomas Jefferson commissioned Meriwether Lewis and William Clark to lead an expedition to explore the newly acquired Louisiana Territory in 1803. Lewis and Clark assembled a group of 30 men to travel up the Missouri River to map the territory and establish trade relations with Native American tribes. After two years of exploration, they reached the Pacific Ocean and successfully returned, having mapped over 8,000 miles of territory and making contact with dozens of tribes. Their expedition doubled the size of the United States and established the nation's presence in the West.