Why did they gang up on the Pawnee?
Sort of a Hutu v Tutsi situation. Lots of inter tribal hatred and warfare for centuries before, during and after Columbus and European expansion. Everyone generally hated the Sioux...the name derived from an indian corruption of French slang for “snake”.
During the Civil War many Indian tribes will side with Confederates and attack white settlers.
Other tribes will support the Union and fight against the Confederates’ allies.
These battles are counted as also Civil War battles, and spread the war to western states & territories that were otherwise untouched.
The Massacre Canyon battle took place in Nebraska on August 5, 1873 near the Republican River. It was one of the last hostilities between the Pawnee and the Sioux (or Lakota) and the last battle/massacre between Great Plains Indians in North America.[2] The massacre occurred when a large Oglala/Brulé Sioux war party of over 1,500 warriors led by Two Strike, Little Wound, and Spotted Tail attacked a band of Pawnee during their summer buffalo hunt. In the ensuing rout more than 150 Pawnees were killed, men with mostly women and children, the victims suffering mutilation and some set on fire.
Cruel and violent warfare like this had been practiced against the Pawnee by the Lakota Sioux for centuries since the mid-1700s and through the 1840s.
http://nebraskastudies.org/1500-1799/emergence-of-historic-tribes/the-pawnee-the-lakota-sioux/
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/