<P> The Louisiana Purchase was by far the largest territorial gain in U.S. history . Stretching from the Mississippi River to the Rocky Mountains, the purchase doubled the size of the United States . Before 1803, Louisiana had been under Spanish control for forty years . Although Spain aided the rebels in the American Revolutionary War, the Spanish didn't want the Americans to settle in their territory . </P> <P> Although the purchase was thought of by some as unjust and unconstitutional, Jefferson determined that his constitutional power to negotiate treaties allowed the purchase of what became fifteen states . In hindsight, the Louisiana Purchase could be considered one of his greatest contributions to the United States . On April 18, 1802, Jefferson penned a letter to United States Ambassador to France Robert Livingston . It was an intentional exhortation to make this supposedly mild diplomat strongly warn the French of their perilous course . The letter began: </P> <P> The cession of Louisiana and the Floridas by Spain to France works most sorely on the U.S. On this subject the Secretary of State has written to you fully . Yet I cannot forbear recurring to it personally, so deep is the impression it makes in my mind . It completely reverses all the political relations of the U.S. and will form a new epoch in our political course . Of all nations of any consideration France is the one which hitherto has offered the fewest points on which we could have any conflict of right, and the most points of a communion of interests . From these causes we have ever looked to her as our natural friend, as one with which we never could have an occasion of difference . Her growth therefore we viewed as our own, her misfortunes ours . There is on the globe one single spot, the possessor of which is our natural and habitual enemy . It is New Orleans, through which the produce of three - eighths of our territory must pass to market, and from its fertility it will ere long yield more than half of our whole produce and contain more than half our inhabitants . France placing herself in that door assumes to us the attitude of defiance . Spain might have retained it quietly for years . Her pacific dispositions, her feeble state, would induce her to increase our facilities there, so that her possession of the place would be hardly felt by us, and it would not perhaps be very long before some circumstance might arise which might make the cession of it to us the price of something of more worth to her . Not so can it ever be in the hands of France . The impetuosity of her temper, the energy and restlessness of her character, placed in a point of eternal friction with us...</P> <P> Jefferson's letter went on with the same heat to a much quoted passage about "the day that France takes possession of New Orleans ." Not only did he say that day would be a low point in France's history, for it would seal America's marriage with the British fleet and nation, but he added, astonishingly, that it would start a massive shipbuilding program . </P>

Who was president when the louisiana purchase was completed