The United States made the Louisiana Purchase from France in 1803 for $15 million.
The agreement transferred roughly 828,000 square miles of territory, much of it in the Mississippi River basin west of the river. The acquisition nearly doubled the nominal size of the United States and secured American control of the strategically important port of New Orleans and the Mississippi River.
President Thomas Jefferson had initially authorized negotiators Robert R. Livingston and James Monroe to purchase New Orleans and nearby areas. French Treasury Minister François Barbé-Marbois unexpectedly offered the entire Louisiana Territory instead. The treaty was signed in Paris on April 30, 1803, and announced in the United States on July 4.
The deal was politically controversial because Jefferson questioned whether the Constitution clearly authorized territorial purchases. It also did not mean that France had settled or governed every part of the territory; many Native nations lived there and retained their own political systems. The purchase remains one of the largest territorial expansions in United States history.