In the chivington attacked a cheyenne village where the cheyenne flew an american flag.

The answer is: the U.S. troops, formed mostly by Colorado volunteers leaded by Colonel John M. Chivington.

Also known as Sand Creek Massacre, it took place in southeastern Colorado teritory on November 29, 1864. It was a controversial surprise attack upon a Cheyenne Indian camp, they had already surrended and were dissarmed. The U.S. force was formed by about 675 troops. Although the Cheyenne chief, Black Kettle had been trying to negotiate the peace, they were attacked. The Cheyenne raised the U.S. flag and the white flag but were still massacred, from 150 to 200 indians were killed. This massacre had influence and was a cause of the Apaho-Cheyenne war.

Sure, I've made a drawing earlier today which you can use if you want.

(It may be a little fuzzy because it's been compressed into a file.)