Before 1860, Native American tribes were regarded as independent nations, but their relationship with the Americans consisted of many broken promises. Many Indians were sent to reservations as a result of the "Conservation" policy in 1851. Congress eventually repealed this and put them on other reservations that were barely better, but the Native Americans were hurt more when the Americans would hunt their buffalo.
The ecological changes that the white settlers brought, along with their hunting, destroyed millions of buffalo, taking away the main source of Native American food. This made the natives feel as if they needed to fight to preserve their way of life.
The natives fought back against the American settlers, trying to preserve their way of life. At Sand Creek, a volunteer militia slaughtered 133 unsuspecting Native Americans, 105 of them women and children. At Little Bighorn, the most famous battle between the two groups, the natives killed every American, having the biggest Native American army assembled at one time. Black Kettle was able to escape, though.
Natives were also threatened by white vigilantes, like Bill Cody, who participated in "Indian Hunting". Killing Indians became a sport, and people would be rewarded bounties for bringing back scalps and skulls of Indians, using this sport as an excuse to eliminate the tribes. The Native Americans still kept hope, though, participating in a new revival, called the "Ghost Dance" led by Wovoka (a Paiute), which inspired visions of white people leaving and buffalo being restored. Chief Joseph was a prominent Indian leader as well.
The Indian tribes would continue to be destroyed, though, and at Wounded Knee, the Seventh Cavalry tried to round up a group of starving Sioux, but this started a conflict, killing 40 whites and 300 of the Indians initially. This led to a massacre of the Indians, and they were mowed down by American cannons in the snow.
The federal government would now step in and end the Native Americans for good. They passed the Dawes Act in 1887, allowing for the gradual elimination of native owned land and distributing it to individual owners. This law forced Native Americans to assimilate, taking away some of their land and even their children.