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 in Dakota Territory July 28–29, 1864. The location of the battleground is in modern Dunn County, North Dakota. With a total of more than 4,000 soldiers involved in the total operation, and more than 2,000 in the battle, Sully's expedition was the largest ever carried out by the U.S. army against Native Americans. Background In the aftermath of the Dakota War of 1862, the U.S. government punished the Sioux, including those who had not participated in the war. Large military expeditions into Dakota Territory in 1863 pushed most of the Sioux to the western side of the Missouri River at least temporarily and made safer, although not entirely safe, the frontier of white settlement in Minnesota and the Dakotas. Four whites were killed by Sioux raiders in the spring of 1864. An important impetus to another military campaign against the Sioux was the desire to protect lines of communication with recently discovered goldfields in Montana and Idaho. The lifeline for the American gold miners were steamboats plying the Missouri River through the heart of Sioux territory. During the winter of 1863-1864, Sully's superior, Major General John Pope ordered Sully to establish several forts along the Missouri River and in the eastern Dakotas to secure the communication routes to the goldfields and to eliminate the Sioux threat to the settlers east of the Missouri River. Sully's First Brigade, consisting of up to 1,700 men, followed the Missouri River from its starting point at Sioux City, Iowa. The Second Brigade with about 1,550 men would march overland from Fort Ridgely in Minnesota. On the march up the Missouri, the Sioux killed one soldier and wounded another. The three Sioux perpetrators were caught, killed, and decapitated. Additional soldiers and civilians with 15 steamboats chugged up the Missouri River to support the army on the ground. Sully's two columns of soldiers united on June 29 and on July 7 he established Fort Rice on the Missouri River in North Dakota as a base, supplied by steamboat, for his military expedition. His scouts, Winnebago and friendly Sioux and mixed-bloods, informed him of a large encampment of Sioux 130 miles northwest near the Little Missouri River. On July 19, he and his men departed Fort Rice to search for the Sioux encampment.