The "Concentration" policy of 1851 gave each tribe its own defined reservation. This divided the tribes and made them easier to control.
The buffalo had supported the tribe's way of life. Hunting and ecological changes accompanying white settlement decimated the population. This destroyed the Indian's source of food and supply, so they couldn't resist white advance. Indian resistance flared.
At the Sand Creek Massacre, a band under Chief Black Kettle camped near Fort Lyon for peace and protection from fighting, Bbut, Colonel Chivington killed many of his people anyway.
Little Bighorn 1876: The Sioux rose up and left the reservation. They resisted white officials and tribal warriors surprised Custer and killed his regiment.
White vigilantes began tracking and killing Indians for sport. This was often in service to eliminate tribe or for rewards. This was called "Indian Hunting".
Starving and fading, many Indians turned to a prophet named Wovoka who inspired a spiritual awakening.A mass called the "Ghost Dance" inspired visions of the white leaving the plains and restoration of the buffalo herds.
Wounded Knee: Dec. 29 1890, Seventh Cavalry tried to round up 350 Sioux, but wound up killing them.
I see the whites leaving!
The Dawes Act of 1887 provided for the gradual elimination of tribal ownership of land and the allotment of tracts to individual owners. The Bureau of Indian Affairs tried to move Indian families on their own plot of land.