Third Treaty of San Ildefonso

The Third Treaty of San Ildefonso, signed on 1 October 1800, was a secret agreement between Spain and France under which Spain retroceded the Louisiana territory to French control in exchange for Bourbon dynastic concessions in Italy. The transfer remained largely unannounced until Spanish officials formally handed over New Orleans in 1803, shortly before France sold the territory to the United States in the Louisiana Purchase. It set in motion the chain of events that moved a vast swath of North America out of Spanish hands within a few years.