The Battle of Killdeer Mountain (also known as the Battle of Tahkahokuty Mountain) took place during Brig. Gen. Alfred Sully’s expedition against the Sioux Indians in Dakota Territory on June 28, 1864. The location of the battleground is in modern Dunn County, North Dakota. With a total of more than 4,000 soldiers involved, Sully’s expedition was the largest ever carried out by the U.S. army against Indians.