What significant purchase doubled the size of the United States in 1803?

The Louisiana Purchase was a monumental event in American history that doubled the size of the United States in 1803. This acquisition involved the United States purchasing approximately 828,000 square miles of territory from France for about $15 million. It not only expanded the geographical boundaries of the nation but also provided vast resources and opportunities for exploration and settlement.

The significance of the Louisiana Purchase lies in its establishment of a crucial foundation for the nation's westward expansion, known as Manifest Destiny, as it opened up vast lands for farming, commerce, and exploration. It also played a pivotal role in the geopolitical dynamics of North America, reducing French territorial claims on the continent and increasing American influence.

Investing in such a large swath of land allowed the U.S. to develop economically and politically, setting the stage for future growth and conflict over these new territories. The other options listed, while important, involved smaller territorial adjustments that did not provide the same expansive opportunities as the Louisiana Purchase.