By 1870, only one group of American Indians in the 300,000 square miles of the Dakota and Montana Territories still held firm against being placed on reservations: a few thousand Teton Sioux and Northern Cheyennes, all followers of the charismatic Sitting Bull. It was then that Philadelphia's Jay Cooke, "the financier of the Civil War," a man who believed that he was "God's chosen instrument," funded a second transcontinental railroad. This line, the Northern Pacific, would follow the Yellowstone River through Montana, separating the last buffalo herds from Sitting Bull's people...