The Assiniboines claimed the land on which Fort Union was built. This tribe occupied both sides of the border with Canada and thus had a choice of trading with either the Hudson’s Bay Company or the American Fur Company. The Crow Indians lived on the upper Yellowstone River and its tributaries. This was a pleasant, bountiful land, and the Crow were considered the richest tribe east of the Rocky Mountains.
Farther up the Missouri, the Blackfeet also claimed land on both sides of the international boundary. Since the days of Lewis and Clark, when a Blackfoot warrior had been killed by the exploration party, these Indians considered American whites their enemies. The Blackfeet, however, welcomed British traders in their midst. Bourgeois Kenneth McKenzie took advantage of this situation when a trapper named Jacob Berger wandered into Fort Union. Berger had worked for the British and had learned the Blackfeet language. McKenzie sent the man to the Blackfeet with an invitation to visit the fort. The scheme worked and the Blackfeet eventually allowed the American Fur Company to establish a trading post in their territory.
The Indians were shrewd bargainers and the trading companies were in fierce and constant competition. Rarely did any of these tribes threaten Fort Union with violence. Occasionally, a disgruntled leader gathered his followers and attempted a takeover. None achieved success. Violence often did occur among the Indians themselves, especially at trading time. Every year the traders smuggled alcohol into the upper Missouri country, despite laws to the contrary. Many trading sessions concluded with the Indians becoming thoroughly intoxicated and settling old scores with one another
Liquor was not the only scourge the traders introduced to the upper Missouri. In 1837 the steamboat St. Peters arrived at Fort Union bringing with it smallpox. The disease struck the fort’s employees just when a band of Assiniboine arrived to trade. Fort traders went out to meet them, taking along trade goods and urging the Indians not to approach any closer. The Assiniboine paid no heed. Their bodies had little resistance against the foreign virus, and of the approximately 1,000 people in the band who caught the disease, only about 150 survived. Other bands kept coming in that summer and smallpox spread throughout the tribe. The Blackfeet were also ravaged by the disease. The Crow Indians, somehow, escaped the pestilence. Twenty years later, in 1857, smallpox struck again. The Assiniboine suffered once more and, this time, the disease swept through the Crow tribe, striking down young and old alike.
The first Sioux appeared in the vicinity of Fort Union in 1847. Before this, they had lived farther downstream, but white expansion from the east forced them to roam westward in increasing numbers. By the 1860s their hostility towards whites, and even other Indian tribes, made them a menace to life at Fort Union. After the Minnesota uprising of 1862, the U.S. Army undertook inconclusive campaigns against the Sioux. This led to the establishment of Fort Buford a short distance downriver from Fort Union. The Sioux continued to harass both forts and anyone traveling between them. Fort Union was finally abandoned and dismantled in 1867. The romance of the fur trade on the high plains and in the Rocky Mountains was now but a memory.