The Louisiana Purchase was the 1803 acquisition that nearly doubled the size of the United States.
The United States agreed to buy the Louisiana Territory from France for $15 million. The treaty was signed in Paris on April 30, 1803, and the purchase was publicly announced in the United States on July 4. The territory covered about 828,000 square miles, including land west of the Mississippi River and the important port of New Orleans.
President Thomas Jefferson wanted secure access to the Mississippi River and New Orleans for American commerce. The purchase also removed the possibility that France would control a large region next to the expanding United States. Jefferson had constitutional concerns about whether the federal government clearly possessed authority to acquire territory, but he supported the agreement.
The acquisition included land that later became part or all of fifteen present U.S. states and parts of two Canadian provinces. Its boundaries and ownership were clarified through later exploration, treaties, and territorial organization.