On this date in 1804, the French flag was lowered in St. Louis, soon-to-be-MO, and the U.S. flag was raised.
The Spanish lieutenant governor for French northern Louisiana handed the keys over to Amos Stoddard, acting as the proxy for the French government, who then handed them to himself, since he was also the newly-appointed U.S. military commander of the territory.
Napoleonic-era geopolitics were nothing if not interesting. See, France had lost the territory to Spain in a war, but Napoleon got it back in exchange for a promise not to kick Spain's ass. However, the deal was all hush-hush, and so the French left the Spanish administration in place so that nobody would suspect anything. Then they went and sold the house with the renters still in it, so to speak. This led to a business boom for surveyors and an honest-to-Spooner strip of ungoverned No-Man's-Land for the next 20 years, as Madrid and Washington argued about on whose side of the property line the hedge was planted.