In 1801, Spain had signed a secret treaty, with France, to give back the territory known as Louisiana. Reports of the treaty reached America, causing much unease amongst the Americans who had been settling further and further west, as much of their supplies came up the Mississippi River, through New Orleans.
American leaders were so worried about the situation, that they began considering an alliance with England. At the same time as that was being discussed, a special envoy was sent to France, to negotiate for the purchase of New Orleans.
On April 11, 1803 the French surprised the Americans by asking straight out, what the US would pay for all of Louisiana. Negotiations were quick, and the US bought the whole 828,000 square miles of Louisiana for $15 million. That price amounted to less than three cents an acre.
Thirteen new states would eventually come out of the area.