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. It is one of the earliest declared National Historic Landmarks of the United States. The fort, perhaps first known as Fort Henry, 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.