The Louisiana region once encompassed an area much larger than the present state. It referred to the area west of the Mississippi River, which was drained by the Red, Arkansas, and Missouri rivers — a huge land of more than 800,000 square miles. Louisiana had been claimed for France by LaSalle in 1682 and named to honor Louis XIV. It became a formal French colony in 1731 and remained as such until it was ceded to Spain in 1762.
In 1800, Napoleon Bonaparte negotiated the Treaty of San Ildefonso with Spain, through which France was given back control of the territory it had ceded in 1763. Spain in compensation received the new kingdom of Etruria in Tuscany, which went to the Duke of Parma, son-in-law of the King of Spain. In 1801, President Thomas Jefferson received word that Louisiana was about to be returned to French control. This was a matter of concern to the president. Spain was a declining power and offered no threat to the United States. Such was not the case with France under Napoleon. James Monroe was dispatched to Paris to supplement the American mission led by Robert Livingston. Monroe was instructed to try to purchase New Orleans and Florida. The former was vitally important to westerners because it controlled the outlet into the Gulf of Mexico.
Before Monroe arrived, the French foreign minister Talleyrand startled Livingston by offering to sell the entirety of Louisiana. Napoleon was at war with Britain and thought it unwise to court distraction by having to defend New Orleans. Monroe helped to complete the deal after his arrival, settling on a price of $11.25 million, plus the cancellation of American claims against the French in the amount of $3.75 million, for a total of $15 million. The treaty was signed on April 30, 1803.
Jefferson was astonished when presented with the details of the purchase. He had severe doubts about the constitutionality of acquiring land through purchase because the Constitution did not address that issue. However, he feared that Napoleon would change his mind if America waited to ratify a constitutional amendment. Demonstrating great flexibility, Jefferson ignored party considerations and submitted the treaty to the Senate where it was overwhelmingly ratified on October 20, 1803.
The Spanish reminded the French that their treaty expressly forbade France from alienating the territory to any third party, but Napoleon didn't care and the Spanish were in no position to assert their rights.