The Louisiana Purchase was the 1803 land deal that nearly doubled the size of the United States.
The United States acquired the territory from France for $15 million. The agreement covered about 828,000 square miles west of the Mississippi River, including land that later became all or part of fifteen states and two Canadian provinces. President Thomas Jefferson authorized the purchase, and the Senate ratified the treaty in 1803.
France’s decision was shaped by its difficulties in Saint-Domingue and by renewed war with Britain. The purchase gave the United States control of the Mississippi River and the port of New Orleans, both vital to western settlement and trade.
The deal’s constitutional basis was debated because the Constitution did not explicitly describe a power to acquire foreign territory. The Lewis and Clark Expedition, launched in 1804, explored much of the new region. The purchase was not the same as the later acquisitions of Alaska or land from Mexico.