The 1864 campaign grew from two impulses: revenge and greed. The Dakota War of 1862 had left Minnesota settlers dead and the federal government determined to punish the Sioux, even those who had not participated in the uprising. At the same time, gold strikes in Montana and Idaho created urgent demand for protected supply routes. Steamboats plying the Missouri River through Sioux territory became the lifeline for American miners. Major General John Pope ordered Sully to establish forts along the Missouri and eliminate the Sioux threat. Fort Rice rose on the river in early July. From there, Sully led 2,200 men westward. On July 28, at the Battle of Killdeer Mountain, he defeated 1,600 Sioux warriors. The survivors fled into the badlands, and Sully followed.

The Dakota badlands offered the Sioux their best chance. Deep, impassable ravines cut the landscape. High rugged hills blocked lines of sight and movement. Water was scarce. The terrain near present-day Theodore Roosevelt National Park became a natural fortress. Sully knew the risks but pressed forward anyway. Two steamboats loaded with supplies waited for him on the Yellowstone River, fifty miles to the north. His only path led through the badlands. A Blackfoot scout promised he knew a route passable by wagon train. On August 5, Sully followed the Heart River upstream and entered the maze. With him came not just soldiers but an emigrant wagon train carrying miners, their wives, and their children into the unknown.

The Sioux struck on August 7. A small raiding party hit the horse herd of the Seventh Iowa Cavalry and ambushed one company. Hundreds of warriors appeared on hilltops near Sully's camp. Cannon fire dispersed them, but the soldiers spent an anxious night. The next morning, a thousand warriors materialized on the bluffs and hills at Sully's front and flanks. Arrows rained down on the column, strung out over three or four miles in the broken terrain. Sully responded with artillery and cavalry sallies. The assault was more harassment than commitment. The Sioux knew they did not need to overwhelm the soldiers. They needed to slow them down, keep them from water, let thirst and exhaustion do the killing. Near the end of the day, a bullet found Sully's Blackfoot guide.

Despite constant opposition, Sully's column advanced ten miles on August 8. The next day brought more warriors, more arrows, more harassment. Then, around noon on August 9, the terrain opened. Sully broke out of the badlands onto a large, level plain. With room to maneuver and deploy his artillery properly, he quickly dispersed the Indians. The battle was over. Sully found the remains of a large, recently vacated Indian camp. The Sioux had scattered in all directions. He estimated a hundred Indians killed, though that number was almost certainly exaggerated given the long range of most exchanges. His own casualties were light: the wounded Blackfoot scout and about a dozen soldiers.

Escaping the badlands did not end the ordeal. Fifty miles of parched desert lay between Sully and the Yellowstone River. His men marched on short rations with only a pint of alkaline coffee each day. Horses and mules died of thirst in large numbers. On August 12, the survivors reached the Yellowstone and found the supply steamboats waiting. Relief was short-lived. At Fort Union, where the Yellowstone meets the Missouri, Sully discovered the Sioux had stampeded the fort's entire horse herd, leaving only two animals. With no horses and exhausted men, he abandoned his plan to continue the campaign. The Sully expedition had pushed the Sioux west of the Missouri into the Powder River country and Black Hills. The army would send another large force in 1865, but the Sioux would resist for another decade.