The Louisiana Purchase (1803) from France resulted in the eventual transfer of about 828,000 square miles of land in North America to the United States. At a cost of about $15 million, the United States nearly doubled its territory and effectively secured control over the Mississippi River by acquiring the port of New Orleans and the vast watershed beyond the river. Beginning in the mid-sixteenth century the Frenchof the territory running from the Great Lakes to the Gulf Coast, only to cede much of it to Spain after the Seven Years’ War (1756–1763). In 1784, Spain closed the Mississippi to American commerce until the Treaty of San Lorenzo (1795). Five years later, however, the territory reverted to France while remaining under Spanish administration. The new U.S. president, , inaugurated in 1801, spent the next two years negotiating long-term access to the river. Jefferson worried that war in Europe might leave the British in control of Louisiana or even that American settlers might establish new nation-states in the Mississippi valley and negotiate their own treaty with the French. The crisis worsened in 1802 when the Spanish closed the river again; some in the opposition Federalist Party suggested the United States go to war. Then, in 1803, the French suddenly offered to sell imperial rights to all of Louisiana. Jefferson seized upon this unexpected opportunity, but worried that the U.S. Constitution did not allow such a deal. Still, he called Congress to an early session and laid the treaty before the Senate to ratify, which they did. The acquisition of so much territory eventually strained the union between North and South and helped to bring on the (1861–1865). Unplanned and unexpected, the Louisiana Purchase presented the federal government and the American people with an array of new challenges and new opportunities.