The government enacted the "Concentration Policy" in 1851 in order to make Indians easier to control & to give the whites more land. However, the plan failed which lead to the Indian Peace Commission (1867) that intended to move all Indians to two large reservations: Oklahoma & Dakotas.
In the 1850s, whites had been killing buffalo at an exponential rate, effectively removing an Indian custom a food source for their own self benefit
The Little Bighorn Battle (1876) involved 2,500 Indians being rounded up in resistance, only to be dispersed & defeated by General George A. Custer. The Sand Creek Massacre (1864) involved "Black Kettle" & his men by an Indian safe camp, being attacked by drunken men, 133 were killed.