The significant event in U.S. history involving the sale of a vast territory in 1803 is the Louisiana Purchase. The United States, under President Thomas Jefferson, acquired approximately 828,000 square miles of territory from France for $15 million. This purchase doubled the size of the nation and opened vast lands for exploration and settlement. The acquisition had profound implications for America’s expansionist ambitions, allowing for westward movement and the fulfillment of the Manifest Destiny ideology. It also raised questions about governance and the future of slavery in the new territories, setting the stage for conflicts in the years to come.