In 1789, the western border of the United States was the Mississippi River. Beyond the river lay the large Spanish territory of Louisiana.

The Mississippi River had become the western border of the English colonies in North America as a result of the Treaty of Paris, which had ended the French and Indian War in 1763. When the United States won the War for Independence, the river became its western boundary, although the British retained navigational rights to the river.

At this time in the young country's history, the majority of the population still lived east of the Appalachian Mountains. Some settlements had been established in the Ohio River Valley and various other parts west of the mountains, but for the most part there were few white Americans living in this region.

The Mississippi River would remain the western boundary of the United States until 1803. In that year, the country bought the Louisiana Territory from France, which had recently acquired it from Spain. This was the largest single territorial acquisition in the country's history. With this purchase of land, the United States' western border was moved over one thousand miles further west to the Rocky Mountains.

It should be noted that in 1789, while the United States considered the Mississippi River to be the entire western frontier of the country, Spain lay claim to parts of western Florida and considered a section of the Chattahoochee River to be the boundary between the two territories.