The Black Dakotas

A government agent with the authority to enact a generous peace treaty with the Sioux (including thousands of dollars in gold) is killed en route to the Dakotas and impersonated by Brock Marsh (Gary Merrill), a Confederate sympathizer. Marsh and his allies are intent on getting their hands on the gold to support the Southern war effort, even if a betrayal of the Sioux results in a devastating war on the local citizens.