Fort Ellis was an early United States Army outpost established in 1867 to the eastern side of present-day Bozeman, Montana as settlers moved into the Gallatin Valley. It was home to the Second Cavalry and Yellowstone explorer Gustavus C. Doane. The United States Army is the largest branch of the armed forces of the United States. ... Bozeman is a city in southwestern Montana, USA. It is the county seat of Gallatin County. ... The Gallatin River The Gallatin River is a tributary of the Missouri River, approximately 120 mi (193 km long), in the U.S. states of Wyoming and Montana. ... Yellowstone National Park is a U.S. National Park located in the states of Idaho, Montana, and Wyoming. ...