What was the Louisiana Purchase?

The Louisiana Purchase was the 1803 acquisition of about 828,000 square miles of French-claimed territory between the Mississippi River and the Rocky Mountains, doubling the size of the United States overnight and giving the new nation control of the entire Mississippi watershed. Napoleon Bonaparte had reacquired the Louisiana territory from Spain in the secret Treaty of San Ildefonso on October 1, 1800, planning to rebuild a French empire in North America centered on the sugar colony of Saint-Domingue (modern Haiti). The plan collapsed when the Haitian Revolution led by Toussaint Louverture and Jean-Jacques Dessalines defeated French armies between 1801 and 1803, killing tens of thousands of French soldiers including General Charles Leclerc, Napoleon's brother-in-law. With Britain about to renew war and yellow fever destroying his Caribbean forces, Napoleon abandoned the American empire scheme and decided to sell.

President Thomas Jefferson had sent Robert Livingston to Paris in 1801 and dispatched James Monroe in 1803 with instructions to buy New Orleans and West Florida for up to 10 million dollars, fearing that French control of New Orleans would strangle American commerce. To their astonishment, French foreign minister Charles Maurice de Talleyrand offered the entire Louisiana territory. Livingston and Monroe accepted on the spot and signed the treaty on April 30, 1803, although the formal purchase documents are dated May 2, 1803. The price was 15 million dollars (about 50 million dollars in 1803 currency or roughly 415 million today), of which 11.25 million was for the land and 3.75 million was assumption of French debts owed to American citizens. The total worked out to less than three cents per acre.

Jefferson faced a constitutional dilemma. He believed in strict construction of the Constitution and saw no enumerated power to acquire territory, but the Senate ratified the treaty 24 to 7 on October 20, 1803 and Congress appropriated funds. The boundaries were vague: the territory ran from the Mississippi River west to the Rockies and from the Gulf of Mexico north to British Canada, encompassing all or part of what became 15 states (Arkansas, Iowa, Kansas, Louisiana, Minnesota, Missouri, Montana, Nebraska, North Dakota, Oklahoma, South Dakota, Wyoming, and parts of Colorado, New Mexico, and Texas).

To explore and map the new territory, Jefferson dispatched the Lewis and Clark Expedition in May 1804. Spain and the United States later disputed the southwestern boundary; the Adams-Onis Treaty of 1819 settled it. The Louisiana Purchase opened the trans-Mississippi West to American settlement and made expansion to the Pacific feasible.