Before he even knew the Louisiana Purchase would take place, President Thomas Jefferson (on June 20, 1803) charged his private secretary, Meriwether Lewis, with the task of exploring the "country of Louisiana." Lewis, a Captain in the U.S. Army, had a specific Presidential mandate:
The object of your mission is to explore the Missouri river; & such principal stream of it, as by it’s course & communication with the waters of the Pacific ocean, may offer the most direct & practicable water communication across this continent for the purposes of commerce.
The land Lewis would explore was ultimately part of the Louisiana Purchase. Jefferson thought there might be a waterway that stretched from the Missouri River (at St. Louis) to the Pacific Ocean. There actually was no direct waterway, but the assignment Jefferson entrusted to Lewis turned out to be one of the greatest explorations of all time.