Via the Third Treaty of San Ildefonso, Spain cedes Louisiana to France, which would sell the land to the United States thirty months later.

The Third Treaty of San Ildefonso was a secret agreement signed on 1 October 1800 between Spain and the French Republic by which Spain agreed in principle to exchange its North American colony of Louisiana for territories in Tuscany. The terms were later confirmed by the March 1801 Treaty of Aranjuez.

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 ($7/km²), the United States nominally acquired a total of 828,000 sq mi now in the Central United States. 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.