...or was it Montana? I never was too sure.
Fort Union was only a few miles away from Fort Buford. In fact, in the day they could see each other. Where they differed was Fort Union was a company owned trading post whereas Fort Buford was strictly a military fort.
You can see that in this screenshot even though the Missouri has slipped a bit down since.
Notice the muddy Yellowstone river emptying into the clear Missouri River.