Final answer:
The Louisiana Purchase had the significance of doubling the size of the United States, securing control of the Mississippi River, and opening up new opportunities for westward expansion.
Step-by-step explanation:
The Louisiana Purchase of 1803 had several significant impacts on the United States. First, it doubled the size of the country as the United States acquired a vast amount of territory from France, including nearly the entire Great Plains region of the present-day. Second, it secured control of the Mississippi River and the key port of New Orleans, which were crucial for American agricultural commerce. Lastly, it opened up new opportunities for westward expansion, allowing for the exploration, settlement, and economic development of the newly acquired territories.