Fort Belle Fontaine is a former U.S. military base located in St. Louis County, Missouri, across the Mississippi and Missouri rivers from Alton, Illinois. The fort was the first U.S. military installation west of the Mississippi, in the newly acquired Louisiana Territory, and served as a starting point for many expeditions to the American West.

The Louisiana Purchase was the acquisition of the territory of Louisiana by the United States from the French First Republic in 1803. This consisted of most of the land in the Mississippi River's drainage basin west of the river. In return for fifteen million dollars, or approximately eighteen dollars per square mile, the United States nominally acquired a total of 828,000 sq mi in Middle America. However, France only controlled a small fraction of this area, most of which was inhabited by Native Americans; effectively, for the majority of the area, the United States bought the preemptive right to obtain Indian lands by treaty or by conquest, to the exclusion of other colonial powers.