Colorado War: Colorado volunteers led by Colonel John Chivington massacre at least 150 Cheyenne and Arapaho noncombatants at Sand Creek inside Colorado Territory.

The Colorado War was an Indian War fought in 1864 and 1865 between the Southern Cheyenne, Arapaho, and allied Brulé and Oglala Lakota peoples versus the U.S. Army, Colorado militia, and white settlers in Colorado Territory and adjacent regions. The Kiowa and the Comanche played a minor role in actions that occurred in the southern part of the Territory along the Arkansas River. The Cheyenne, Arapaho, and Lakota Sioux played the major role in actions that occurred north of the Arkansas River and along the South Platte River, the Great Platte River Road, and the eastern portion of the Overland Trail. The United States government and Colorado Territory authorities participated through the 1st Colorado Cavalry Regiment, often called the Colorado volunteers. The war was centered on the Colorado Eastern Plains, extending eastward into Kansas and Nebraska.