“Quickly” is the answer to this question, before Napoleon could change his mind.
In the early 19th century, New Orleans was already a vital port, strategically located at the mouth of the long Mississippi.
Early settlers in the West shipped nearly a third of their produce in flatboats along its tributaries and down the river to the harbor city for shipment abroad. The right to deposit these cargoes was obtained from Spain, which since 1763 had had a rather feeble hold on the massive territory, stretching west to the Rockies and north beyond the present United States–Canadian border.
A shock rippled through the West when news spread that Spain had secretly negotiated the transfer of the territory to France in the Treaty of San Ildefonso (1800). Two years later, the gates of New Orleans suddenly closed on Americans.
President Thomas Jefferson, who was dreaming of America as a quiet, agrarian utopia immune to foreign entanglements, was forced to act. Fearing an outright clash instigated by the irate settlers, he decided to try negotiating with Napoleon.
Robert Livingston and James Monroe were dispatched to Paris with $2 million appropriated by Congress for the purchase of New Orleans and western Florida. The two were rather stunned, therefore, when Talleyrand, representing France, proposed the sale of the entire territory.
A number of reasons lay behind this move: France was on the brink of war with Great Britain and Napoleon feared the British Navy could block his occupation of Louisiana anyway. And after a slave insurrection in Hispaniola proved victorious against his troops, Napoleon decided to focus his attention on Europe.
After no more than a moment’s hesitation, the American diplomats agreed, doubling the size of their country in the blink of an eye. Some haggling occurred over the price, but since the idea was Napoleon’s, “It is not only New Orleans that I will cede,” he had announced; “it is the whole colony without any reservation”, and since he needed money, the purchase price was modest to say the least: $15 million for the valleys of the entire Mississippi and its western tributaries, an area of 828,000 square miles.
“You have made a noble bargain for yourselves,” Talleyrand told the Americans, “and I suppose you will make the most of it.”