3. Louisiana Purchase (1803)

In 1803, President Thomas Jefferson negotiated the Louisiana Purchase from France for $15 million, acquiring roughly 828,000 square miles of territory that doubled the size of the United States overnight. The deal, finalized under Napoleon Bonaparte, included vast lands from the Mississippi River to the Rocky Mountains.

This massive land acquisition fueled westward expansion, promoted Manifest Destiny, and opened the door to economic growth through agriculture and resources.