The United States agreed to pay France $15 million for the Louisiana Purchase in 1803.
The agreement transferred a vast territory west of the Mississippi River to the United States. President Thomas Jefferson had originally sought access to New Orleans and the Mississippi River, but French representatives offered the entire Louisiana territory during negotiations. Robert Livingston and James Monroe represented the United States, while François Barbé-Marbois negotiated for France.
The purchase covered approximately 828,000 square miles and nearly doubled the nominal size of the United States. It included land that later became parts or all of fifteen present-day states, as well as small areas now in Canada. The United States also assumed claims and diplomatic complications involving Native nations and neighboring colonial powers.
The deal was controversial at home because Jefferson questioned whether the Constitution clearly authorized such an acquisition. Congress nevertheless approved the arrangement. The United States took formal possession in 1803, and the Lewis and Clark Expedition soon helped survey and describe the newly acquired region.