Final answer:
On December 7, 1941, Japan launched a surprise attack on Pearl Harbor, prompting the United States to enter World War II.
Explanation:
On December 7, 1941, Japanese fighter planes attacked the United States military base at Pearl Harbor in Hawaii, leading to significant damage and casualties. This event prompted President Roosevelt to declare it as 'a date which will live in infamy' and request a declaration of war, resulting in the United States entering World War II.
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