Fort Union was the leading fur trading post on the upper Missouri River from 1829 to 1867, serving as the American Fur Company’s headquarters in the region and linking the Upper Missouri country with St. Louis. Built by the Upper Missouri Outfit under Kenneth McKenzie, it became a major center of exchange where Assiniboine, Crow, Cree, Ojibwe, Blackfoot, Hidatsa, Lakota, and other tribes traded buffalo robes and furs for manufactured goods. As trade shifted from beaver pelts to bison robes, the post grew in economic and political importance, shaping relations with Indian nations and U.S. influence near the northern border until soldiers dismantled it in 1867.