While the Indians of the western mountains accommodated the arrival of white miners, trappers, and missionaries, the Plains Indians largely maintained their traditional ways during the first years of white ingress.

The first trail across the northern United States was the Oregon Trail. To protect travelers along its passage through Wyoming, the U.S. government produced the Fort Laramie Treaty in 1851, which was signed by the Crow, Gros Ventre, and Assiniboin. These tribes were assigned reservations in eastern Montana, as were the Blackfeet, who did not attend the meeting or sign the treaty, but were assigned a reservation in absentia.

The discovery of gold in the Rocky Mountains increased the demand for transportation routes across treaty Indian country. The Bozeman Trail, blazed during the 1860s, cut across Sioux tribal land to reach Montana’s goldfields. Three military forts were built to protect the trail. Gold was also discovered in the Black Hills of South Dakota, country considered sacred by the Sioux, and prospectors flooded in.

These infractions by the whites infuriated the Indians. The U.S. government responded by unilaterally diminishing the size of the original reservations. The Sioux and Cheyenne, among the last of the tribes to be forced into Montana by white western expansion, were especially angry at the ongoing incursions. After the gold rush in the Black Hills, the Sioux quit the reservation completely and resumed their traditional plains lifestyle on the prairies of eastern Montana.