The basic cause of the Indian Wars between 1860 and 1890 was the fact that the Americans wanted the land that the Indians had and the resources that might be in and on that land. The earliest wars were brought on to some degree by the Civil War, but the basic cause was still the American desire for more land and resources.
As American pioneers moved west, they encountered Indian tribes. The Americans wanted the land that belonged to the Indians and so the Indians needed to be moved. This movement was generally accomplished by signing treaties with the Indians whereby the Indians agreed (some historians argue that they did this without understanding what they were signing and/or under duress) to move to specific areas and leave the rest of the land for the whites. However, as time went by, pioneers wanted more and more land. This was particularly true when there were gold rushes like the one in Colorado in 1859 or the one that followed the discovery of gold in the Black Hills in 1874. When this happened, the Indians had to be moved onto smaller and smaller reservations. When the Indians were forced to move, conflict often erupted, usually between the Americans and Indian factions that did not want to live on the reservations. The main cause of these wars, then, was the Americans’ desire for more land and resources.
The main effects of these wars was to push the Indians off their land and on to reservations and, thereby, to open up the land to American settlers. As the Indians were pushed onto reservations, they lost their ability to live their traditional way of life. The loss of their traditional ways was hurried by things like the destruction of the bison, the Dawes Severalty Act, and the boarding school system. Once the Indians were removed, the American settlers were free to enjoy the use of the land.
Thus, the Indian Wars of 1860 to 1890 were caused by the American desire for more land, and the wars resulted in the destruction of the Indians’ way of life and the opening of the West for American use.