San Ildefonso, Treaty of, any of several treaties signed at the royal residence of San Ildefonso, Spain. 1 The Treaty of San Ildefonso of 1796 was an alliance of France with Spain against Great Britain in the French Revolutionary Wars. 2 The secret Treaty of San Ildefonso of 1800 was actually a draft confirmed by two later treaties (Mar., 1801; Oct., 1802). Spain retroceded Louisiana to France and was compensated by the creation in Tuscany of the kingdom of Etruria, which was given to the duke of Parma, son-in-law of Charles IV of Spain. Although Napoleon Bonaparte agreed never to alienate Louisiana, he disregarded the treaty and sold Louisiana to the United States three years later.
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