Fort Union Trading Post National Historic Site is the site of a partially reconstructed trading post on the Missouri River and the North Dakota/Montana border, twenty-five miles from Williston, North Dakota. It is one of the earliest declared National Historic Landmarks in the United States. The fort, possibly first known as Fort Henry or Fort Floyd, was built in 1828 or 1829 by the Upper Missouri Outfit managed by Kenneth McKenzie and capitalized by John Jacob Astor's American Fur Company.