Intertribal Trade

"The Arikara, Mandan and Hidatsa were all middlemen who participated in a trading network that stretched across western North America. Nomadic groups traded the horses they had acquired from the Spanish Southwest, the French and British goods they had received from Canada, and the Euro-American goods they had obtained from the expanding St. Louis-based fur trade for the corn, beans, squash, and pumpkins these groups grew near their semi-sedentary earth-lodge villages"

From Trailtribes.org

By the time the fur trade finally declined in the 1870's, the cultures of the Plains [Indians] had been transformed beyond recognition. The enduring legacy of the fur trade was to leave the Indians in a state of chronic dependency that persists to the present day. Ultimately, by destroying the animal resources of their environment, it eroded not only their traditional economy but also their very way of life, forcing them to leave hunting behind for a sedentary existence on reserves.

According to p. 92 of this book, it was "...on the foundation of an extensive intertribal trade that the white man built up...commerce" in the early days of exploration into the continent. I've often come across references to a long-standing rivalry or feud between the Ojibwa and Sioux. It was in large part as a result of new alliances of some tribes with the French, and some with the English, which in turn made it profitable for some to foster hostilities between old foes (in this case the Ojibwa) and new allies (Sioux).