The Battle of Powder River, also known as the Reynolds Battle, occurred on Friday, March 17, 1876, in Montana Territory, United States. The attack on a Cheyenne Indian encampment by Colonel Joseph J. Reynolds initiated the Great Sioux War of 1876. Although destroying a large amount of Indian property, the attack was poorly carried out and probably solidified Lakota Sioux and northern Cheyenne resistance to the U.S. attempt to force them to sell the Black Hills and live on a reservation.