When westward expansion really got into full swing in the mid-1800s, the US government felt the need to do something to ensure that the white settlers would be able to take the land they wanted without interference from Indian tribes. The government adopted a policy of putting Native Americans on reservations and backed this policy up with military force. They also killed the buffalo. These things resulted in the removal of Native Americans from most of the Great Plains.
White settlers moving into the Great Plains needed large tracts of land to farm. They needed to be sure that Native Americans would not attack them. They also needed to be sure that huge herds of buffalo would not destroy their crops. For these things, they turned to the US government.
The US government, then, started to make deals with Indian tribes, paying them and promising them support if they would stay on reservations. Tribes typically agreed to these deals, though it is not clear to what extent they did so voluntarily. If the tribes did not agree, or if they left the reservations, the US Army defeated them in the Indian Wars.
In addition, the government helped facilitate the killing of the buffalo. This helped prevent the Indians from practicing their old way of life and made them dependent on the government for food.
In these ways, government policies and the Indian Wars combined to force the Indians off the Great Plains.
Government policy and the Indian Wars worked together to remove the Native Americans from the Great Plains.
On the one hand, there were government policies that were meant to help move white settlers onto to the land. Some examples of this were the Homestead Act and the laws that subsidized the building of railroads.
Along with this came the Indian Wars. These wars were "necessary" in order to allow settlement to occur. As settlers came in, conflict inevitably happened between them and the Indians. The Indian Wars, then, were waged to protect the settlers and then to push Indians on to reservations (that kept shrinking) so as to solidify white control of the Great Plains.