Final answer:
The large tract of land bought by the United States that doubled its size is known as the Louisiana Purchase, completed in 1803 under President Thomas Jefferson for $15 million.
Step-by-step explanation:
The Louisiana Purchase
The large tract of land that the United States purchased that doubled the size of the country was known as the Louisiana Purchase. In 1803, President Thomas Jefferson secured this tremendous acquisition from France for $15 million. The Louisiana Purchase not only provided vast amounts of new land for agriculture and settlement but also had significant implications for the expansion of slavery, the displacement of Native American tribes, and the broadening of U.S. commercial interests.
Jefferson's vision for an "empire of liberty" was greatly enhanced with this acquisition, enabling the growth of the United States as an agrarian republic. Moreover, important geographical features like the Mississippi River and the port of New Orleans became crucial to the burgeoning American agricultural commerce. This landmark deal is often regarded as perhaps the greatest real estate deal in American history.