In this book, Barton Barbour presents the first comprehensive history of Fort Union, the nineteenth century's most important and longest-lived Upper Missouri River fur trading post. Barbour explores the economic, social, legal, cultural, and political significance of the fort which was the brainchild of Kenneth McKenzie and Pierre Chouteau, Jr., and a part of John Jacob Astor's fur trade empire.From 1830 to 1867, Fort Union symbolized the power of New York and St. Louis, and later, St. Paul merchants' capital in the West. The most lucrative post on the northern plains, Fort Union affected national relations with a number of native tribes, such as the Assiniboine, Cree, Crow, Sioux, and Blackfeet. It also influenced American interactions with Great Britain, whose powerful Hudson's Bay Company competed for Upper Missouri furs.

Barbour shows how Indians, mixed-bloods, Hispanic-, African-, Anglo-, and other Euro-Americans living at Fort Union created a system of community law that helped maintain their unique frontier society. Many visiting artists and scientists produced a magnificent graphic and verbal record of events and people at the post, but the old-time world of fur traders and Indians collapsed during the Civil War when political winds shifted in favor of Lincoln's Republican Party.

In 1865 Chouteau lost his trade license and sold Fort Union to new operators, who had little interest in maintaining the post's former culture.