"That her father had bought land from the Indians might not help at all, for another Indian could always be found to dispute the right of the original Indian to sell the land at all." (from Flint, by Louis L'Amour)
This illustrates the difficulties of dealing with "Indian lands." Some of the tribes settled on lands, built homes and farmed the land. However, many of the plains tribes were basically nomadic. How do you "own" land if all you do is travel across it occasionally to hunt?