Final answer:
Robert Livingston and James Monroe signed the Louisiana Purchase Treaty on behalf of the United States in 1803, which doubled the size of the nation.
Step-by-step explanation:
The Louisiana Purchase was a landmark real estate deal between the United States and France in which the United States acquired approximately 827,000 square miles of land west of the Mississippi River. Thomas Jefferson, who was the President of the United States at that time, orchestrated the purchase but faced a constitutional dilemma as there was no provision for acquiring new land in the Constitution. The two men who signed the treaty for the Louisiana Purchase on behalf of the United States were Robert Livingston and James Monroe. The agreement was signed in Paris on April 30, 1803, and the purchase was finalized later that year, significantly expanding the size of the United States.