The Louisiana Purchase was the 1803 acquisition that nearly doubled the size of the United States.
The United States purchased the Louisiana Territory from France for $15 million. President Thomas Jefferson authorized negotiations mainly to secure New Orleans and navigation rights on the Mississippi River, but France offered the entire territory instead. The agreement was signed on April 30, 1803, and the transfer was completed later that year.
The territory covered about 828,000 square miles west of the Mississippi River. It included land that later became all or parts of 15 present-day U.S. states and two Canadian provinces. The purchase extended U.S. control westward to the Rocky Mountains and strengthened access to major inland waterways.
The transaction raised constitutional questions because the Constitution did not explicitly describe a presidential power to acquire foreign territory. Jefferson nevertheless supported the deal, and the Senate approved the treaty. The Louisiana Purchase is separate from the Alaska Purchase of 1867, which involved Russia.