Final answer:
Thomas Jefferson negotiated the purchase of the Louisiana Territory from France in 1803, which doubled the size of the United States and secured critical trading routes such as the port of New Orleans and the Mississippi River.
Step-by-step explanation:
When Napoleon of France took control of the Louisiana Territory, President Thomas Jefferson did not support this change in control. Instead, Jefferson saw an opportunity and negotiated the purchase of the entire Louisiana Territory from France. The port city of New Orleans and control over the Mississippi River were vital for the United States, especially for trade and westward expansion.
Despite concerns over the constitutional authority to make such a purchase, the agrarian vision that Jefferson cherished, and the existing political and economic circumstances, prompted him to appoint Robert Livingston and James Monroe to negotiate with France. In 1803, the Louisiana Purchase Treaty was signed, effectively doubling the size of the United States for the sum of $15 million, or mere pennies per acre.