The Louisiana Purchase transferred the territory to the United States in 1803.
The United States negotiated the purchase with France during President Thomas Jefferson’s administration. The agreement was signed on April 30, 1803, and the United States formally took possession later that year. The purchase price was $15 million.
The territory covered about 530 million acres west of the Mississippi River, stretching from the Mississippi basin toward the Rocky Mountains. It approximately doubled the land area of the United States and gave the young nation control of the major inland port of New Orleans.
The purchase raised constitutional questions because the Constitution did not expressly describe a power to acquire foreign territory. Jefferson nevertheless supported the agreement, and the Senate ratified the treaty in October 1803. The acquisition led to exploration, settlement, and long-running disputes with Indigenous nations and other powers.