In early 1801, President Thomas Jefferson learned that Spain had ceded the Louisiana Territory back to France and, fearing Napoleon would close the Mississippi to American trade, sent Ambassador Robert Livingston to Paris in September 1801 to warn that the United States would not tolerate losing its trade outlet or having French colonies on its borders. In 1802, as Napoleon prepared to occupy New Orleans with troops, Jefferson sent Pierre du Pont de Nemours to France to tell the French that the United States would ally with England if they annexed Louisiana, and then dispatched James Monroe to Paris with authority to buy New Orleans and West Florida for $9,375,000. Monroe and Livingston were instructed to negotiate peacefully or, if France resisted, form an immediate alliance with England. Although settlement seemed unlikely, Napoleon announced on April 11, 1803, that he would cede the entire territory to the United States. By April 30, 1803, a treaty had been signed for the purchase of the full 830,000 square mile region for $15,000,000, at less than three cents per acre, a transaction that shaped the destiny of the United States, ended Napoleon's dreams of a French empire, and confirmed Spain's fears of America's westward expansion.