Final answer:
The Louisiana Purchase had a profound impact on the United States by doubling the country's size, and facilitating westward expansion and trade, while also introducing challenges such as slavery debates and the displacement of Native Americans.
Step-by-step explanation:
Based on the excerpt about the October 20, 1803 ratification of the Louisiana Purchase Treaty, it can be inferred that the purchase would have significant effects on the United States. First, by doubling the size of the country, it set the stage for significant westward expansion beyond the Mississippi River. The affordable land acquisition, at approximately four cents per acre, marked one of the greatest real estate deals in American history, supporting Thomas Jefferson's vision of an agrarian republic. It also opened up opportunities for trade, particularly through the crucial ports of New Orleans and the Mississippi River, facilitating American agricultural commerce. However, the acquisition also brought challenges such as the debate over slavery in new territories, the displacement of Native American tribes, and constitutional questions regarding presidential powers to make such purchases.