The Natives of the American West were badly affected by the US Army. The Army destroyed their ability to live in their traditional ways. The Army did not do this because it was evil, but because that was what most Americans wanted it to do.
Before white settlers came west, the Army left Indians in the West more or less alone. However, white settlers soon came west and wanted the lands the Indians lived on. The US government tried to give the settlers what they wanted and had the Army work to move the Indians off the lands that the settlers wanted.
In order to do this, the Army fought many battles against Native Americans. They killed many Indians in these wars. They also pushed the Native Americans off their lands and forced them to live on reservations. By doing so, they made it impossible for the Indians to live in their traditional ways.
In these ways, we can see that the Army affected Native Americans very deeply. The Army (acting as Americans wanted it to) killed many Native Americans and forced the rest to live on reservations where they could not practice their traditional ways of life.