Lewis knew that he could not lead such a great expedition alone, so he wrote to his old friend William Clark asking him to join the expedition as co-captain. In September 1803, Clark agreed to join Lewis almost instantly. Although the U.S. army refused the promotion of Lieutenant Clark, Lewis still considered him as his equal throughout the entire expedition (Huntington 2). Lewis and Clark hired twenty one soldiers and twenty frontiersmen, and proudly called these men the Corps of Discovery (Gunderson 8-9). Along with the Corps of Discovery, Clark brought his slave, York. Although York was forced to come along, he was of great use to the expedition and did things no African-American had done before such as carrying a gun. York was also most likely the first African-American to vote in the U.S. because he took part in the Corps voting on where to stay the winter ("History and Culture: People" 7). .
In the two years before the expedition, a lot of preparations were made. Lewis studied about the Native American tribes, medicine, geology, and astronomy. Because he would be making the first maps of the land, Lewis had to learn to navigate by using the stars. To make peace with the native peoples they would meet, Lewis also purchased the following gifts: medals, flags, fabrics, and beads. He also had a boat specifically built for the expedition (Gunderson 6-7, "Lewis and Clark Expedition 1). Jefferson warned Lewis that " 'if there's any threat of violence or difficulties, pull back instead of forcing ahead. If you have to give up the whole mission, it's worth it. Don't put yourselves or the native peoples in danger'" (qtd in Huntington 3). On May 14th, 1804 Lewis and Clark launched their boats into the Missouri River at St. Louis; along with them their forty two men (Gunderson 10-12). "Lewis and Clark Expedition" states that Lewis purchased a dog for twenty dollars, named him Seaman, and brought him along (1).