Answer :

Back in after the civil war European settlers wanted to expand out west for land to live. But Indians had tribes and land that covered the whole United States at the time! The natives way of life changed with Dawes act! An act that took land from natives, sent them to reservations, and basically destroying there culture and tribal strength. For many tribes were split up.
The Europeans pushed the Indians west and moved them onto reservations and tried to assimilated them to be like the Europeans. Trail of tears is an example of this. During westward expansion the American citizens killed off nearly all the buffalo which was a major source of food for the Great Plains Indians. Lots of disagreement between the two cultures. American Indians also contracted new diseases from the settlers which killed off most of their populations and there were also many diseases in the new world that the settlers were not immune to either and killed much of them. Lots of fighting as well!