What were the circumstances surrounding the Louisiana Purchase?

The Louisiana Purchase stemmed from a long-standing desire on the part of the United States to gain control of the Mississippi River, or at least the city of New Orleans. Without unlimited access to New Orleans, American expansion was compromised, since farmers west of the Appalachians had no way to ship their crops to market (because trans-Appalachian rivers flowed toward the Mississippi, and there was as yet no canal linking the Great Lakes to the Hudson River). The port city had been in Spanish hands, but it, along with the vast territory of Louisiana, came under French control in 1802. Alarmed by this development, which President Thomas Jefferson thought might arouse the ambitions of Napoleon, James Monroe went to France with a mandate to secure New Orleans and West Florida for the United States. In the meantime, however, the Haitian Revolution led to that island nation's independence from France. This meant that the port city of New Orleans was no longer all that useful to the French. They also feared war with Great Britain, and believed that Louisiana would fall into the hands of that nation.

These developments changed France's outlook. They offered Monroe and ambassador Robert Livingston an astonishing bargain--the whole of the Louisiana Territory, encompassing most of the modern United States from the Mississippi to the Rockies--for $15 million. Monroe agreed to the Purchase, but Jefferson was conflicted, since he doubted its constitutionality. He had, after all, long advocated a philosophy of strict constructionism in matters involving the Constitution. Eventually, however, the Purchase was approved by Jefferson and the Senate. Jefferson, having already ordered the exploration of the American Northwest (including much of the territory in question) by the Lewis and Clark expedition, had through a series of fortuitous events, vastly increased the size of the United States, setting in motion a half-century of extraordinary territorial growth that saw the nation expand its borders to the Pacific.