Located on the upper Missouri River on the present-day Montana--North Dakota border, Fort Union was the nineteenth century's most important and longest-surviving fur trading post.  American Indians and people of many races created a system of community law that sustained this frontier society.  Scientists, artists, researchers, and missionaries were frequent visitors.  Fort Union affected U.S. relations with Great Britain, whose powerful Hudson's Bay Company competed for furs on the upper Missouri, and with the Blackfeet, Crow, Cree, and other Indian tribes.  In this comprehensive history, Barton H. Barbour explores Fort Union's economic, legal, political, and sociocultural importance.