In 1803, the United States purchased the Louisiana Territory, an area that included most of South Dakota, from Napoleon Bonaparte, and President Thomas Jefferson organized a group commonly referred to as the "Lewis and Clark Expedition" to explore the region. In 1817, an American fur trading post was set up at present-day Fort Pierre, beginning continuous American settlement of the area. In 1855, the U.S. Army bought Fort Pierre but abandoned it in 1857 in favor of Fort Randall to the south. Settlement by Americans and Europeans was by this time increasing rapidly, and in 1858 the Yankton Sioux signed the 1858 Treaty, ceding most of present-day eastern South Dakota to the United States.