The Louisiana Purchase in 1803 was significant because it effectively doubled the size of the United States, acquiring approximately 828,000 square miles of territory from France for $15 million. This land deal, negotiated by President Thomas Jefferson, provided an opportunity for westward expansion and the exploration of the continent. It opened up vast resources and land for agriculture, spurring economic growth and settling by American pioneers. The Purchase also raised questions about the extension of slavery into the new territories, setting the stage for future conflicts that would eventually contribute to the Civil War.