Perhaps the greatest contribution of Thomas Jefferson's administration was the Louisiana Purchase. Jefferson did not come into office with the desire to expand the nation. On October 1, 1800, Spain ceded the Louisiana Territory to France in the Treaty of San Ildefonso. The territory was equal in size to the entire United States at the time. Napoleon Bonaparte envisioned a Caribbean empire, with the Louisiana Territory providing the resources to support the center of the empire on the island of Santo Domingo (now Haiti). At the time the Treaty of San Ildefonso was signed, Santo Domingo was controlled by former slaves, under Toussaint L'Ouverture, who had driven their masters from the island. Napoleon dispatched the French army to regain control of the island, but the islanders met the troops with fierce resistance. Faced with this resistance, and many troops suffering from yellow fever, the French retreated in defeat. Napoleon gave up on his plan for a Caribbean empire.
By 1802, France had still not taken control of the Louisiana Territory, leaving it in the hands of the Spanish despite the fact that the land belonged to France. In October 1802, the Spanish colonial administrator in New Orleans prohibited American crops from being deposited at the port of New Orleans before being shipped to other nations. This severely constricted US commerce in the southwest, and many Americans believed, incorrectly, that the order had actually come from Napoleon. Fears of French control of the Louisiana Territory, and especially of New Orleans, loomed large. Jefferson began efforts to ingratiate himself to the British in preparation fro enlisting their aid against the French.
Jefferson sent James Monroe and Robert Livingston to France with the intention of negotiating the purchase of the port of New Orleans, in an attempt to end, at long last, American difficulties there. He also instructed them to negotiate the purchase, if possible, of as much of Florida as possible. However, the envoy found Napoleon had given up on his plan for a Caribbean empire in order to focus on the war in Europe. In order to finance French efforts in Europe, he wanted to sell all of the Louisiana Territory. After some negotiation, the price was set at $15 million in April 1803, for which the US gained an enormous, uncharted piece of land to the west of the Mississippi River. For the price of approximately 13.5 cents per acre, the United States had doubled its size.
Jefferson, always the strict constructionist, feared that the purchase would be deemed unconstitutional. Therefore, he personally drafted a constitutional amendment authorizing the national government to acquire new lands and allowing for the indefinite settlement of the new territory. However, Jefferson and his colleagues feared the time it would take to adopt a new amendment might allow the deal to slip through their fingers. Urged by fellow Republicans, he dropped the amendment and submitted the treaty that provided for the Louisiana Purchase to the Senate, where it was speedily ratified.
Jefferson had long imagined an "empire of liberty" that would span North America, and perhaps even extend into South America. However, he did not enter office with any clear plans for expansion. With Spain, an increasingly weak power, in control of the Louisiana Territory, Jefferson reasoned that it was only a matter of time until the US would have an opportunity to expand westward. However, once the Louisiana Territory changed hands, the situation changed. Jefferson did not trust France, or more specifically Bonaparte, to stay out of North American affairs. He feared the nation would find itself wedged between Britain in Canada and France in Louisiana, a weaker player in a North American geographical struggle dominated by the world's two largest powers.
When the Spanish closed the port of New Orleans to American crops, Jefferson was forced to act in defense of American interests. He sought only to end the longstanding quarrels over New Orleans and counterbalance French advances in North America by purchasing parts of Florida. However, Napoleon presented the US with another option, which, while Jefferson had not seriously considered it, fascinated him. In the end, it proved a wise decision to purchase the Louisiana Territory. The nation doubled its geographical size, gained access to the resources of the wilderness and important waterways for travel and commerce. Additionally, the Louisiana Territory served as a sort of buffer zone, keeping foreign powers in North America at a greater distance from the primary population of the United States.