What Was the Lewis and Clark Expedition?By: Judith St. George, Who HQ Illustrator: Tim Foley Series: What Was? When Meriwether Lewis, William Clark, and the "Corp of Discovery" left St. Louis, Missouri, on May 21, 1804, their mission was to explore the vast, unknown territory acquired a year earlier in the Louisiana Purchase. The travelers hoped to find a waterway that crossed the western half of the United States. They didn't. However, young readers will love this true life adventure tale