The massacre at Sand Creek was an attack by

In 1864, Colorado militia attacked a camp of Cheyenne and Arapaho people at Sand Creek, many of whom were peaceful and had been told they would be safe. The attack is known as the Sand Creek Massacre. So the correct choice is a Colorado militia on peaceful American Indians; it was not an attack on Dakota warriors or an attack by federal soldiers.

AI-written and checked against the verified answer.