http://en.wikipedia.org/wiki/Plains_Indians

There were government initiatives at the federal and local level to starve the population of the Plains Indians by killing off their main food source, the bison. The Government promoted bison hunting for various reasons: to allow ranchers to range their cattle without competition from other bovines and to weaken the Plains Indian population and pressure them to remain on reservations.[13] The herds formed the basis of the economies of local Plains tribes of Native Americans for whom the bison were a primary food source. Without bison, the Native Americans would be forced to leave or starve.