It's a dark reality, but during the colonizing and settling of North America, 90% of native Americans died from disease (yes, it was unknowingly brought by Europeans before they knew what germs were) so the Native Americans didn't need much land for their remaining tribes. All of the land was purchased by the U.S. from the controlling tribes. (keep in mind, Indian tribes viciously fought each other for hunting grounds and territory and the land changed hands many times prior to U.S. purchasing it)
