The Yellowstone expedition was an expedition to the American frontier in 1819 and 1820 authorized by United States Secretary of War John C. Calhoun, with the goal of establishing a military fort or outpost at the mouth of the Yellowstone River in present-day North Dakota. Sometimes called the Atkinson–Long Expedition after its two principal leaders, Colonel Henry Atkinson and Major Stephen Harriman Long, it led to the creation of Fort Atkinson in present-day Nebraska, the first United States Army post established west of the Missouri River, but was otherwise a costly failure, stalling near Council Bluffs, Iowa.