One of Jefferson's acts doubled the area of the country. At the end of the Seven Years' War, France had ceded to Spain the territory west of the Mississippi River, with the port of New Orleans near its mouth -- a port indispensable for the shipment of American products from the Ohio and Mississippi valleys. Shortly after Jefferson became president, Napoleon forced a weak Spanish government to cede the great tract called Louisiana back to France. The move filled Americans with apprehension and indignation. Napoleon's plans for a huge colonial empire just west of the United States threatened the trading rights and the safety of all American interior settlements. Jefferson asserted that if France took possession of Louisiana, "from that moment we must marry ourselves to the British fleet and nation." Napoleon, knowing that another war with Great Britain was impending, resolved to fill his treasury and put Louisiana beyond the reach of the British by selling it to the United States. This put Jefferson in a constitutional quandary: the Constitution gave no office the power to purchase territory. At first Jefferson wanted to amend the Constitution, but his advisers told him that delay might lead Napoleon to change his mind -- and that the power to purchase territory was inherent in the power to make treaties. Jefferson relented, saying that "the good sense of our country will correct the evil of loose construction when it shall produce ill effects."  

For $15 million, the United States obtained the "Louisiana Purchase" in 1803. It contained more than 2,600,000 square kilometers as well as the port of New Orleans. The nation had gained a sweep of rich plains, mountains, forests and river systems that within 80 years would become the nation's heartland -- and one of the world's great granaries.  

As Jefferson began his second term in 1805, he declared American neutrality during the struggle between Great Britain and France. Although both sides sought to restrict neutral shipping to the other, British control of the seas made its interdiction and seizure much more serious than any actions by Napoleonic France.  

By 1807 the British had built their navy to more than 700 warships manned by nearly 150,000 sailors and marines. The massive force controlled the sea lanes: blockading French ports, protecting British commerce and maintaining the crucial links to Britain's colonies. Yet the men of the British fleet lived under such harsh conditions that it was impossible to obtain crews by free enlistment. Many sailors deserted and found refuge on U.S. vessels. In these circumstances, British officers regarded it as their right to search American ships and take off British subjects, to the great humiliation of the Americans. Moreover, British officers frequently impressed American seamen into their service.  

When Jefferson issued a proclamation ordering British warships to leave U.S. territorial waters, the British reacted by impressing more sailors. Jefferson decided to rely on economic pressure to force the British to back down. In December 1807 Congress passed the Embargo Act, forbidding all foreign commerce. Ironically, the Republicans, the champions of limited government, had passed a law that vastly increased the powers of the national government. In a single year American exports fell to one-fifth of their former volume. Shipping interests were almost ruined by the measure, and discontent rose in New England and New York. Agricultural interests found that they too were suffering heavily, for prices dropped drastically when the Southern and Western farmers could not export their surplus grain, cotton, meat and tobacco.  

The hope that the embargo would starve Great Britain into a change of policy failed. As the grumbling at home increased, Jefferson turned to a milder measure, which conciliated domestic shipping interests. In early 1809 he signed the Non-Intercourse Act permitting commerce with all countries except Britain or France and their dependencies.  

James Madison succeeded Jefferson as president in 1809. Relations with Great Britain grew worse, and the two countries moved rapidly toward war. The president laid before Congress a detailed report, showing several thousand instances in which the British had impressed American citizens. In addition, northwestern settlers had suffered from attacks by Indians whom they believed had been incited by British agents in Canada. This led many Americans to favor conquest of Canada. Success in such an endeavor would eliminate British influence among the Indians and open up new lands for colonization. The desire to conquer Canada, coupled with deep resentment over impressment of sailors, generated war fervor, and in 1812 the United States declared war on Britain.