Kenneth McKenzie built Fort Union in 1828 or 1829 for the Upper Missouri Outfit, backed by John Jacob Astor's money and his near-total monopoly on the American fur trade. The location was strategic: two miles from where the Yellowstone joins the Missouri, close enough to the northern border to assert American sovereignty, and positioned at the crossroads of tribal territories. Assiniboine, Crow, Cree, Ojibwe, Blackfoot, Hidatsa, and Lakota all came here to trade. In the early years, beaver pelts drove the economy, feeding demand for fashionable beaver hats in New York and Paris. When silk and wool hats became popular in the 1830s, the trade shifted to bison robes, and the post kept thriving.

Fort Union stocked everything the tribes wanted and the traders could profit from: manufactured glass beads, clay pipes, blankets, knives, cookware, cloth, firearms, and alcohol. Northern Plains Indians particularly favored the English-made North West Gun, a smooth-bore flintlock prized for its reliability. The tribes used these weapons to hunt the buffalo whose robes would return to the fort as payment. Father Pierre-Jean De Smet, a Jesuit missionary, passed through and left accounts of the commerce. Captain Joseph LaBarge navigated steamboats up the Missouri carrying trade goods and bringing furs back to St. Louis. The entrepreneurial power concentrated here influenced government policies affecting every tribe in the region.

The fort operated in relative peace for decades, with conflicts between Euro-American traders and Native peoples less common here than clashes among the tribes themselves. That changed in the summer of 1863, following the Dakota Wars of 1862 in Minnesota. Tribes along the upper Missouri turned openly hostile. Fort Union came under something approaching siege, and steamboats running supplies up the Missouri faced genuine danger. Within a few years, the fur trade was dying anyway. In 1867, the Army purchased the aging post and dismantled much of it, hauling the seasoned timber two miles downstream to build Fort Buford. The empire Astor had built was ending, replaced by a military presence that would dominate the region for the next three decades.

In 1961, the Department of Interior designated the site one of the earliest National Historic Landmarks in the United States. The National Park Service named it Fort Union Trading Post specifically to distinguish it from Fort Union National Monument, an Army post in New Mexico. What stands today is a partial reconstruction based on archaeological excavations and the detailed drawings Rudolf Kurz made while working as clerk. The site interprets how portions of the fort may have looked in 1851, at the height of the bison robe trade. Walking through the reconstructed palisade walls, you can understand why this spot commanded the upper Missouri: it offered a natural harbor for steamboats, protection from flooding, and sightlines in every direction across the open plains.

The confluence of the Missouri and Yellowstone rivers remains one of the great geographic landmarks of the Northern Plains. The trading post sat just two miles from where these waters merge, 25 miles from present-day Williston, North Dakota. For the Assiniboine, Crow, and Blackfoot who traveled here to trade, and for the merchants who grew wealthy on their labor, this was the economic heart of the northwestern frontier. Today the reconstructed fort stands as testimony to an era when beaver felt hats drove global commerce and a Scotsman named McKenzie built an empire from buffalo robes traded in the shadow of the Rockies.