The Sand Creek Massacre happened in 1864 in what is now Colorado. This was an incident in which something in the area of 200 to 400 (this depends on what source you read) Cheyenne Indians were killed in cold blood by a group of militia led by Col. J. M. Chivington.
The Cheyenne were being pushed out of Colorado because gold had been discovered in the area of Pikes Peak. The Indians resisted, but the band of Cheyennes that Chivington and his men attacked were camped peacefully and were not doing anything to warrant having been killed. This massacre is seen as one of the worst in the history of white-Indian interactions.