Guardhouse at historic Fort Yellowstone in Yellowstone National Park The fort was a U.S. Army post established in 1891 at Mammoth Hot Springs in Yellowstone National Park. Yellowstone had been designated as a national park in 1872, but the Interior Department was unable to effectively manage the vast and mountainous area. Besides the buildings of Fort Yellowstone, the army left a legacy of policies and practices that served as precedents for the future National Park Service management of national parks. The army's military commanders implemented backcountry patrols, wildlife protection and management, and protection of natural features. Photo by Carol Highsmith.