Jackson County was home to members of the Osage Native American tribe. The first known European explorers were French trappers who used the Missouri River as a highway for explorations and trading with Native American tribes. Jackson County was a part of New France, until the British victory in the French and Indian War in 1763 resulted in the cession of this territory to Great Britain's ally, Spain. Spain was forced by the Third Treaty of San Ildefonso in 1800 to return its Louisiana Territory (of which modern Jackson County then formed a part) to France, which in turn sold it to the United States in the Louisiana Purchase of 1803.