The Pawnee tribe had fought these other tribes for years, and so the Army turned to the Pawnee for help against a common enemy.
The Pawnee became scouts. They were very successful in helping protect the railroad as it was being built across Nebraska, and they accompanied several U.S. Amy expeditions against the warring Sioux, Cheyenne, and Arapaho. But, by the late 1870s, the Pawnee Scouts were disbanded. The U.S. Government had removed most members of the Pawnee tribe from Nebraska to Indian Territory south of Nebraska.
http://www.nebraskastudies.org/1850-1874/native-american-settlers/conflict-among-the-tribes/
Thank you. That explains a lot.