The 1803 agreement through which the United States acquired a vast territory from France was the Louisiana Purchase.
The United States agreed to pay France $15 million, including the assumption of certain claims by American citizens. The purchase covered approximately 827,000 square miles west of the Mississippi River, although the exact boundaries were initially unclear.
President Thomas Jefferson authorized the deal, which was negotiated by James Monroe and Robert R. Livingston with French representative François Barbé-Marbois. The territory effectively doubled the size of the United States and gave the country control of the Mississippi River and the port of New Orleans.
The acquisition raised constitutional questions because the Constitution did not explicitly describe a power to purchase foreign territory. Jefferson accepted the deal, and the Senate ratified the treaty in October 1803. Later exploration included the Lewis and Clark expedition.