Lewis and Clark were American explorers who led the first expedition to cross the western portion of the United States from 1804 to 1806. Commissioned by President Thomas Jefferson, their mission was to explore the newly acquired Louisiana Territory and find a water route to the Pacific Ocean. The expedition, known as the Corps of Discovery, traveled over 8,000 miles, mapping the land and documenting the plants, animals, and Native American tribes they encountered. Their journey provided valuable information about the geography and resources of the western territories, paving the way for future exploration and settlement.