On the last day of April in 1803, the United States pretty much doubled in size as Robert Livingston and future-President James Monroe put pen to paper in Paris and completed the purchase of the Louisiana Territory. The acquisition, the largest single territorial expansion in the history of the country, was the culmination of several years of work and intense debate.
Originally, then-President Thomas Jefferson had asked the French about purchasing just New Orleans. The French leader, expansionist-minded Napoleon Bonaparte, had been formulating designs on an empire in North America for some time. But the failure of his brother-in-law’s attempt to take Saint-Dominique (modern-day Haiti) caused him to rethink his plans. In addition, the idea of unloading the territory to the United States had merit because it would create yet another potential rival to Britain…and Napoleon was all in favor of that.
So rather than simply selling a city, he sold a bunch of wilderness. The United States got a vast new territory with tons of opportunity. President Jefferson, though highly concerned about the Constitutionality of the purchase and faced with a lot of opposition in Congress, ended up with an Midwest-sized feather in his cap, bought for pennies an acre. France got an infusion of cash, an elimination of debts it owed the U.S., and smaller house-keeping bills.
And on March 10, 1804 (almost a year after the official documents were signed), the Louisiana Territory was formally transferred to the United States at a ceremony held in St. Louis, through which the Lewis and Clark Expedition had passed just six months prior.
Recommended Reading: Undaunted Courage: Meriwether Lewis, Thomas Jefferson and the Opening of the American West – Ambrose at his best. An absolute must-read.