Three Flags Day

France had ruled Louisiana from its founding until the Treaty of Paris (1763), which ended the Seven Years' War (whose North American phase was the French and Indian War), in which treaty Spain received the French land west of the Mississippi River (the "right bank" going downstream) plus New Orleans, and Great Britain received the French lands east of the River (the "left bank") -- which included what had previously been called the Illinois Country or Upper Louisiana.

On October 1, 1800, Napoleon Bonaparte concluded France's re-acquisition of La Louisiane (Spanish: Luisiana) from Spain, in the Third Treaty of San Ildefonso.

The U.S. and France agreed on April 30, 1803, to the American purchase of Louisiana (which was announced publicly in the United States on July 4).

However, the U.S. did not immediately take possession of these lands on the west side of the Mississippi, and Spain continued to administer the territory because it had not yet formally turned it over to France.

Lewis and Clark were to spend the winter of 1803–1804 at Camp Dubois in what was then the Indiana Territory, opposite the confluence of the Missouri and Mississippi Rivers.