The significance of the Louisiana Purchase in U.S. history lies in its profound impact on the nation's territorial expansion and economic development. Acquired from France in 1803 for $15 million, the Purchase doubled the size of the United States, adding approximately 827,000 square miles of land. This vast expanse provided opportunities for westward expansion, exploration, and settlement. The Purchase paved the way for the Lewis and Clark expedition, furthering scientific and geographic knowledge of the continent. Additionally, it raised questions about the expansion of slavery into new territories, setting the stage for political tensions that would culminate in the Civil War.