Three Flags Day

Three Flags Day commemorates March 9 and 10, 1804, when Spain officially completed turning over the Louisiana (New Spain) colonial territory to France, who then officially turned over the same lands to the United States, in order to finalize the 1803 Louisiana Purchase.

The ceremony in St. Louis cleared the way for Lewis and Clark to begin their exploration.

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.

Spain officially took control of its territory in 1769, when it suppressed the Rebellion of 1768 by area residents who had resisted Spain's assumption of colonial authority in the formerly French domain.

The United States extended its western boundaries to the Mississippi River during the American Revolutionary War, when General George Rogers Clark took possession of the lands east of the Mississippi River which had for some years belonged to Great Britain. American control of the territory which became today's Midwestern states (the former Illinois Country and Ohio Country) was not secure until both the Treaty of Paris (1763) and the Jay Treaty (1794) had been formalized.

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Wikipedia