Fort Assinniboine rose from the Montana prairie in 1879 amid genuine terror. General Custer had fallen at Little Bighorn just three years earlier. Chief Joseph and his Nez Perce had been captured at Bear Paw, barely forty miles away. Sitting Bull and his Sioux warriors had crossed into Canada, settling in the Cypress Hills, and General Phil Sheridan believed they might sweep back across the border at any moment. The fort that Lt. Col. J.R. Brooke recommended building became massive: 104 buildings sprawling across a military reservation that at its peak encompassed 704,000 acres, stretching from the Missouri River in the south to the Milk River in the north, with the Bear's Paw Mountains rising within its boundaries. More than 750 officers and enlisted men garrisoned here with their families. Yet the dreaded invasion never came. Neither Sitting Bull's Sioux nor the Nez Perce refugees in Canada ever attacked across the border.

Among the units rotating through Fort Assinniboine were the African-American troopers of the 10th Cavalry, known as Buffalo Soldiers. They patrolled the vast reservation, maintaining order during the twilight years of the Indian Wars. When the Spanish-American War erupted in 1898, these same soldiers answered the call. At the Battle of San Juan Hill in Cuba, they supported the flank of Theodore Roosevelt's Rough Riders in the famous charge. Eyewitnesses noted that Roosevelt's volunteers would not have prevailed without the Buffalo Soldiers' critical support. Sergeant Horace Bivens of the 10th Cavalry, who had fought in the Indian Wars of the Southwest and served under Pershing at the fort, earned a Silver Star for valor in Cuba. He later received the first Army Distinguished Pistol Shot badge for his marksmanship scores.

As the years passed and the Indian Wars wound down, an unexpected transformation occurred. Bands of Chippewa and Cree people, displaced from their traditional territories, began drifting onto the massive military reservation. The Cree had fled Canada as refugees following the North-West Rebellion of 1885. The Chippewa had been pushed steadily westward from Wisconsin and Minnesota. Both groups found a precarious sanctuary within the fort's boundaries, trading with the Army and camping on land that belonged to no one but the government. By 1911, when the Army abandoned Fort Assinniboine as obsolete, several hundred Native Americans lived within its grounds. Chief Rocky Boy, whose Chippewa name Asiniiwin meant Stone Child, appealed to the Theodore Roosevelt administration for land and education for his people.

Rocky Boy died just months before Congress acted on his petition, but in 1916 the government ceded a portion of the old fort to create Rocky Boy's Indian Reservation in his honor. It became the smallest reservation in Montana by land area, but it gave the landless Chippewa and Cree peoples a permanent home at last. The abandoned military buildings met a less dignified fate: settlers hauled most of them away for building materials. A handful of structures survived, eventually becoming an agricultural research station for Montana State University and a historic preservation site now listed on the National Register of Historic Places. A portion of the old reservation where Beaver Creek runs through the Bear's Paw Mountains became Beaver Creek Park, the largest county park in the United States at 10,000 acres.