The significance of the Louisiana Purchase for U.S. expansion was monumental, as it doubled the size of the nation in 1803. Acquiring approximately 827,000 square miles from France for $15 million, it secured vast territories for future settlement and economic development. The purchase facilitated westward expansion, shaped Manifest Destiny ideology, and significantly impacted U.S. relations with Native American tribes and other countries, ultimately setting the stage for the nation’s growth and influence.