AFter the mid 1600s (when most of the Eastern Indians died from Old World diseases) Indian power was broken and the white folk got the upper hand. At that point most Indians became professional meat hunters. Still, the problem with disease lingered and their very high infant mortality rate was finally recognized by Jefferson, et al.
The impetus behind the "reservation system" was the need to find a place where Indians could live separate from Old World people so that they could survive. Initially what is now Indiana was selected as a site for a very large reservation. This was later changed to Wisconsin, and finally Oklahoma.
Among others, the Kickapoo Indians were in the lead for the evacuation of Indian people from the East to a safe place in the West.
Modern medicine makes it possible for Indians to live off the reservation.
***The Indians pretty much sold their land for money***
Red Cloud even wanted to sell the Black Hills to the white man. $$$