Final answer:
Japan's attack on Pearl Harbor in 1941 brought the U.S. into World War II.
Explanation:
Japan's attack on Pearl Harbor in 1941 was the significant event that brought the United States into World War II. Prior to this attack, the U.S. had been practicing isolationist policies but the assault on Pearl Harbor led to the U.S. declaring war on Japan and ultimately getting involved in the global conflict.
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