[143] Early in Jefferson's presidency, the administration learned that Spain planned to retrocede the Louisiana territory to France, raising fears of French encroachment on U.S. territory.[144] In 1802, Jefferson and Madison sent Monroe, a sympathetic fellow Virginian, to France to negotiate the purchase of New Orleans, which controlled access to the Mississippi River and thus was immensely important to the farmers of the American frontier. Rather than merely selling New Orleans, Napoleon's government, having already given up on plans to establish a new French empire in the Americas, offered to sell the entire territory of Louisiana. Despite lacking explicit authorization from Jefferson, Monroe, along with Livingston, whom Jefferson had appointed as American's minister to France, negotiated the Louisiana Purchase, in which France sold more than 800,000 square miles (2,100,000 square kilometers) of land in exchange for $15 million ($271 million adjusted for inflation in 2021).[145]
Despite the time-sensitive nature of negotiations with the French, Jefferson was concerned about the constitutionality of the Louisiana Purchase, and he privately favored introducing a constitutional amendment explicitly authorizing Congress to acquire new territories. 