Final answer:
The Louisiana Purchase, approved by Congress in 1803, significantly expanded the United States territory and was a landmark event that doubled the nation's size for $15 million.
Step-by-step explanation:
In 1803, Congress approved the Louisiana Purchase, which added former French territory in the West to the United States. This acquisition was a significant event in American history as it doubled the size of the United States, adding land west of the Mississippi River from the Gulf of Mexico to the Canadian border. The purchase cost the United States $15 million and was considered one of the greatest real estate deals in history. It also provided the United States with vital access to New Orleans and the rest of the interior of North America via the Mississippi River and its tributaries, laying the foundation for further westward expansion and manifesting President Thomas Jefferson's vision of a large agrarian republic.