Louisiana Purchase

Initially Jefferson, through his minister to France, Robert Livingston, offered Napoleon $2 million for a small tract of land on the lower Mississippi. There Americans could build their own seaport. Impatient at the lack of news, Jefferson sent James Monroe to Paris to offer $10 million for New Orleans and West Florida. Almost at the same time, and unbeknownst to Jefferson, France had offered all of Louisiana to Livingston for $15 million.Though the transaction was quickly sealed, there were those who objected to the purchase on the grounds that the Constitution did not provide for purchasing territory. However, Jefferson temporarily set aside his idealism to tell his supporters in Congress that "what is practicable must often control what is pure theory." The majority agreed. Jefferson later admitted that he had stretched his power "till it cracked" in order to buy Louisiana, the largest single land purchase in American history. As a result, generations of Americans for nearly 200 years have been the beneficiaries of Jefferson's noble vision of America and his efforts at expanding the continent.
The Louisiana Purchase is what i think one of Jefferson's Smartest moves he ever made.