The
Third Treaty of San Ildefonso (formally titled the
Preliminary and Secret Treaty between the French Republic and His Catholic Majesty the King of Spain, Concerning the Aggrandizement of His Royal Highness the Infant Duke of Parma in Italy and the Retrocession of Louisiana) was a secretly negotiated treaty between
France and
Spain in which Spain returned the colonial territory of
Louisiana to France. The treaty was concluded on
October 1,
1800 between
Louis Alexandre Berthier representing France and Don
Mariano Luis de Urquijo for Spain. The treaty was negotiated under some duress, as Spain was under pressure from
Napoleon. The terms of the treaty did not specify the boundaries of the territory being returned, which later became a point of contention between Spain and the
United States after the
Louisiana Purchase in
1803, in which France sold its territory to the
United States. This treaty also affirmed the earlier
Treaty of Alliance signed at San Ildefonso on
August 19,
1796. That treaty is sometimes also referred to as the Treaty of San Ildefonso.
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