President Thomas Jefferson commissioned the expedition shortly after the Louisiana Purchase in 1803 to explore and to map the newly acquired territory, to find a practical route across the western half of the continent, and to establish an American presence in this territory before Britain and other European powers tried to claim it.
The campaign's secondary objectives were scientific and economic: to study the area's plants, animal life, and geography, and to establish trade with local Native American tribes. With maps, sketches, and journals in hand, the expedition returned to St. Louis to report its findings to Jefferson.
According to Thomas Jefferson himself, one goal was to find "the most direct and practicable water communication across this continent, for the purposes of commerce." Jefferson also placed special importance on declaring U.S. sovereignty over the land occupied by the many different tribes of Native Americans along the Missouri River, and getting an accurate sense of the resources in the recently completed Louisiana Purchase. The expedition made notable contributions to science, but scientific research was not the main goal of the mission.
During the 19th century, references to Lewis and Clark "scarcely appeared" in history books even during the United States Centennial in 1876 and the expedition was largely forgotten. Lewis and Clark began to gain new attention around the start of the 20th century. Both the 1904 Louisiana Purchase Exposition, in St. Louis, and the 1905 Lewis and Clark Centennial Exposition, in Portland, Oregon, showcased Lewis and Clark as American pioneers. However, the story remained relatively shallow, a celebration of U.S. conquest and personal adventures, until the mid-century, since which time it has been more thoroughly researched and retold in many forms to a growing audience.
In 2004, a complete and reliable set of the expedition's journals was compiled by Gary E. Moulton. In the 2000s, the bicentennial of the expedition further elevated popular interest in Lewis and Clark. As of 1984, no U.S. exploration party was more famous, and no American expedition leaders are more instantly recognizable by name.