The Bozeman Trail ran through the Powder River country, which included the traditional hunting grounds of Lakota, Cheyenne, and Arapaho peoples. Attempts by the natives to prevent encroachment and armed defense of settlers along the trail led to conflicts in short order. Due to the presence of the Sioux in the region, as early as 1864, travelers were advised not to traverse the Bozeman Trail except in very large wagon trains. The US Army also suffered - that year, when a party led by Captain Townshend and several soldiers set out along the Trail with a wagon train, the Sioux attacked his train, killing four soldiers in the assault.

In response to Sioux raids along the Bozeman Trail, the United States Army closed the trail in 1865 to mount the Powder River Expedition against the Sioux alliance that kept ravaging settlers and the beleaguered Crows. With the Civil War nearing its end, spare men were hard to come by, but still, the Powder River Expedition was prepared under the leadership of Brigadier General Patrick Connor.

Charged with keeping the roads and trails of the plains open, Connor’s expedition was war in all but name. Underequipped, and without enough men, the expedition turned out to be little more than a series of limited skirmishes, fortification construction, and requisitions for more men and materiel. Almost from the start, the expedition faced trouble. The various division commanders had a foggy notion of which parts of the Powder River Country they were to march through, with the varied surveys of the region not helping. The biggest problem, however, was the soldiers’ refusal to march. Occurring at the climax of the Civil War, the expedition’s soldiers expected to be discharged and allowed to return to their homes, not stuck in the middle of nowhere, fighting another battle. Dissuaded from mutiny with the helpful aid of artillery, the various divisions finally got underway in July.

The expedition faced vast open country, and that, coupled with lack of supplies, logistics, and communication beyond runners and scouts, quickly took their toll. Men succumbed to scurvy, and the east and middle divisions failed to link up on schedule, thanks largely to the lack of proper surveys of the region and general lack of knowledge of the terrain. This lack of knowledge resulted in supply failures, further exacerbating the expedition’s plight. With the soldiers lacking food in a region sparse of forage for anything except oxen and birds, the natives pounced, attacking the separated divisions.

The natives’ attacks were a rude awakening for the soldiers, as among the three divisions only the Native American scouts had knowledge of the area or experience fighting in the West. Expecting nearly nude savages flinging spears and arrows, the natives’ use of rifles and captured army uniforms took them completely by surprise. Despite the lack of supplies and the Native American raids, the middle and east divisions managed to link up in early September, but as the united divisions marched onward to join with General Connor’s division, 225 horses and mules died from heat exhaustion, starvation, or cold, thanks to a recent mountain storm.

Both the natives’ view of the expedition and General Connor’s offer an idea of the end result. "The Indians, thinking that the commander had voluntarily retired from their front, again hastened to the road, passing General Connor's retiring column to the east of his line of march, and again commenced their devilish work of pillage, plunder, and massacre."

General Connor himself is reported to have stated in regard to the expedition, "You have doubtless noticed the singular termination of the late campaign against the Indians. The truth is, rather harm than good was done, and our troops were, in one sense, driven out of their country by the Indians..."