Final answer:
The Louisiana Purchase doubled the size of the United States and provided new opportunities for westward expansion.
Step-by-step explanation:
The Louisiana Purchase had a significant effect on the United States in 1803. It doubled the size of the country, acquiring about 828,000 square miles of territory from France. The acquisition of this vast land not only expanded the U.S. geographically but also provided new opportunities for westward expansion, exploration, and settlement.
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