The Louisiana Purchase was the 1803 acquisition that nearly doubled the size of the United States.
The United States agreed to purchase the Louisiana Territory from France for $15 million. The treaty 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.
President Thomas Jefferson supported the purchase because control of the Mississippi River and New Orleans was crucial to western farmers and national trade. The deal expanded U.S. territory from the Mississippi River to the Rocky Mountains, although the exact boundaries remained a subject of later negotiation.
The Louisiana Purchase is often confused with the Alaska Purchase of 1867, which added a much smaller but still enormous area in the northwest. The acquisition also preceded the Lewis and Clark expedition, which began in 1804 to survey part of the new territory.