Final answer:
Thomas Jefferson purchased the entire Louisiana Territory from France in the Louisiana Purchase for $15 million, doubling the size of the United States.
Step-by-step explanation:
In 1803, Thomas Jefferson purchased the entire Louisiana Territory from France in what is known as the Louisiana Purchase. The territory had been claimed by Spain and was ceded to France during the Napoleonic Wars. Jefferson believed that control of New Orleans and the Mississippi was important for the United States' access to the interior of the continent. He assigned Robert Livingston and James Monroe to negotiate the purchase with Napoleon, who was motivated to sell due to his change in plans. The terms of the purchase included the United States paying $15 million, which amounted to a few pennies per acre, and gaining enough territory to double the size of the country.