Louisiana Purchase, vast region in North America purchased by the United States from France in 1803. Some 2,100,000 sq km (more than 800,000 sq mi) in area, the territory comprised present-day Arkansas, Missouri, Iowa, Minnesota west of the Mississippi River, North Dakota, South Dakota, Nebraska, Oklahoma, nearly all of Kansas, the portions of Montana, Wyoming, and Colorado east of the Rocky Mountains, and Louisiana west of the Mississippi River but including New Orleans.

The huge province of Louisiana was originally settled by the French in the early 18th century, but in 1762 the area west of the Mississippi and the “Isle of Orleans” was ceded to Spain by a secret treaty, and in 1763, at the end of the Seven Years' War, the area east of the Mississippi was lost to Great Britain. In 1800, by another secret treaty (forced on Spain by Napoleon I), it was returned to France. In 1802 two acts were committed that President Thomas Jefferson regarded as hostile to the interests of the United States: French forces were sent to New Orleans and to Santo Domingo, Hispaniola (now the Dominican Republic), to quell a rebellion there, and the right of deposit, the privilege previously accorded US merchants of depositing goods at New Orleans pending trans-shipment, was withdrawn. Jefferson thereupon sent the American statesman James Monroe to Paris to aid the American minister to France, Robert R. Livingston, in an attempt to effect one of four possible plans advantageous to the United States: (1) the purchase of eastern and western Florida and New Orleans; (2) the purchase of New Orleans alone; (3) the purchase of land on the eastern bank of the Mississippi River to build an American port; or (4) the acquisition of perpetual rights of navigation and deposit.

The previous negotiations between Livingston and Charles Maurice de Talleyrand-Périgord, the French minister of foreign affairs, had been unsuccessful. Subsequently the international situation worsened for France. The French army in Santo Domingo was destroyed by yellow fever and the revolutionary uprising there, and a war with Britain appeared inevitable, threatening occupation of Louisiana by the British. Napoleon, deciding to make the best of an awkward position, gave Talleyrand new instructions, and on April 11, 1803, the foreign minister astonished Monroe and Livingston by offering to sell them all Louisiana or nothing at all. Although operating beyond their authorized power, the American envoys agreed to buy the territory, and early in May the three documents (antedated to April 30) ceding Louisiana to the United States were signed. The price agreed on was $15 million, of which $11,250,000 was to be paid outright by the United States to France. The balance of $3,750,000 was to be paid by the United States to its citizens to satisfy their claims against France.

At the time of purchase, Jefferson was concerned about the constitutionality of making a land acquisition without adding a covering amendment to the US Constitution. The law, however, did give the president treaty-making power, and the Louisiana Purchase was ratified into law as a treaty by the US Senate. The Louisiana Purchase stands as the largest area of territory ever added to the United States at one time.