The historical significance of the Louisiana Purchase, finalized in 1803, was that it doubled the size of the United States, securing vast territories west of the Mississippi River. This acquisition, negotiated by President Thomas Jefferson, involved purchasing approximately 828,000 square miles of land from France for $15 million. The purchase facilitated westward expansion, providing land for agriculture and settlement, and opened new opportunities for trade and exploration. It also raised questions about slavery and governance in new territories. Ultimately, the Louisiana Purchase played a crucial role in shaping the future of the United States and its emergence as a continental power.