The Louisiana Purchase was one of the most colossal land transactions in history. All or parts of 15 Western states would eventually be carved from its nearly 830,000 square miles, which stretched from the Gulf of Mexico to Canada, and from the Mississippi River to the Rocky Mountains. And the price, $15 million, or about four cents an acre, was a breathtaking bargain.

Rich in gold, silver and other ores, as well as endless lands for grazing and farming, the new acquisition made America immensely wealthy.

When Napoléon negotiated the clandestine Treaty of San Ildefonso with Spain’s Charles IV. The treaty called for the return of the territory to France. When Jefferson heard rumors of Napoléon’s secret deal, he immediately saw the threat to America’s Western settlements and its vital outlet to the Gulf of Mexico. Jefferson suspected that Napoléon wanted to close the Mississippi to American use.

In October 1802, Spain’s King Charles IV signed the royal decree transferring the territory to France, The Spanish administrator in New Orleans Morales ended the American right to deposit cargo in the city duty-free. He argued that the three-year term of the 1795 treaty that had granted America this right and free passage through Spanish territory on the Mississippi had expired. As a result, trappers’ pelts, agricultural produce and finished goods risked exposure and theft on open wharfs while awaiting shipment to the East Coast and beyond. The entire economy of America’s Western territories was in jeopardy.

Jefferson had written in April 1802, to the U.S. minister in Paris, Robert R. Livingston, it was crucial that the port of New Orleans remain open and free for American commerce and to try to purchase New Orleans.

In March 1802, Livingston warned that France intended to “have a leading interest in the politics of our western country” and was preparing to send 5,000 to7,000 troops from its Caribbean colony of Saint Domingue (now Haiti) to occupy New Orleans. But Napoléon’s troops in Saint Domingue were being decimated by a revolution and an outbreak of yellow fever. By the time he assembled enough men and ships in January 1803, ice blocked the Dutch port, making it impossible for him to set sail.

That same month Jefferson asked James Monroe to join Livingston in Paris with authority to spend $9,375,000 to purchase New Orleans and parts of the Floridas. By the time Monroe arrived in Paris on April 12, Napoléon had suddenly decided to sell the entire Louisiana Territory to the United States. In Napoléon’s view, the land was useful mainly as a granary for Saint Domingue. With the colony in danger of being lost, the territory was useless.

On April 11, when Livingston called on French foreign minister Talleyrand for what he thought was yet another futile attempt to reach a deal, Talleyrand asked whether the United States would like to buy the whole of the Louisiana Territory. The treaty was signed by on May 2 and backdated to April 30.

Source: How the Louisiana Purchase Changed the World
Smithsonian Institution