Camp Dubois, near present day Hartford, Illinois, served as the winter camp for the Lewis and Clark Expedition from December 12, 1803 to May 14, 1804. It was located on the east side of the Mississippi River so that it was still in United States territory. This was important because the transfer of the Louisiana Purchase to the France from Spain didn't occur until March 9, 1804 and then from France to the United States on March 10, 1804. They returned again to the camp on their return journey on September 23, 1806.

William Clark arrived at Camp Dubois first with a group of men that he recruited from Kaskaskia and Fort Massac on December 12, 1804. Captain Meriwether Lewis joined the camp several weeks later after gathering as much information about Upper Louisiana and the west from Cahokia, Kaskaskia, St. Louis and other locations. Also during this time Lewis took the opportunity to smooth relations with the Spanish authorities to make the transfer of the Louisiana Purchase easier.

Camp Dubois was a fully operating military camp. Soilders stationed at the camp were required to participate in training, maintain personal cleanliness, police the camp and other duties spelled out by the United States military. They had inspections, marched, stood guard duty and hunted to supplement their military rations. Sgt. John Ordway was in charge of the camp during periods in which both Lewis and Clark were away.

On Monday, May 14, 1804, the Expedition, under Clark's command, left Camp River Dubois on the east side of the Mississippi River and sailed up the Missouri River.