[51]

Louisiana Purchase and Minister to Great Britain
Shortly after the end of Monroe's gubernatorial tenure, President Jefferson sent Monroe back to France to assist Ambassador Robert R. Livingston in negotiating the Louisiana Purchase. In the 1800 Treaty of San Ildefonso, France had acquired the territory of Louisiana from Spain; at the time, many in the U.S. believed that France had also acquired West Florida in the same treaty. The American delegation originally sought to acquire West Florida and the city of New Orleans, which controlled the trade of the Mississippi River. Determined to acquire New Orleans even if it meant war with France, Jefferson also authorized Monroe to form an alliance with the British if the French refused to sell the city.[52]
Meeting with François Barbé-Marbois, the French foreign minister, Monroe and Livingston agreed to purchase the entire territory of Louisiana for $15 million; the purchase became known as the Louisiana Purchase. In agreeing to the purchase, Monroe violated his instructions, which had only allowed $9 million for the purchase of New Orleans and West Florida. 