On October 20, 1803, the United States Senate voted by 24 to 7 to ratify the treaty by which France sold Louisiana to the United States, and the largest land purchase in American history was approved. The price was set. It came to about 15 million dollars. The land was huge. The territory covered some 2.1 million square kilometres between the Mississippi river and the Rocky Mountains, which was close to the whole area of the country at the time. The treaty had been signed in Paris on April 30.
The deal came about because Napoleon had failed to re-establish a French empire in the Americas. His army had been destroyed by disease and revolt in Haiti, and he needed money for the war in Europe, and he decided to sell the whole province rather than hold a base at New Orleans. President Thomas Jefferson had sent envoys to Paris to try to buy New Orleans and west Florida, and he was surprised by the offer of the entire territory. He doubted at first that the constitution gave him the power to buy land and add it to the union. The doubt was real.
The Senate agreed to the treaty after some argument about the money and about the people already living in the territory, and the formal transfer was made in December 1803 at New Orleans and in March 1804 at St Louis. The deal was done. The land was largely unexplored by Europeans outside the river towns. Lewis and Clark had been commissioned in 1803 to find a route to the Pacific, and their party set out up the Missouri river in May 1804.
The purchase changed the shape of the country. It opened the way for settlement across the plains and the west, and the land was eventually divided into all or part of fifteen states. It also raised the question of whether slavery would spread into the new territory, a dispute that ran on through the nineteenth century and helped to bring on the civil war. The date of the Senate vote is sometimes given as the moment the purchase took effect.