Answer:While World War II had been raging in Europe since 1939, the United States did not intervene until after Japanese planes bombed Pearl Harbor in 1941. As Japan had an alliance with Germany and Italy, both nations declared war on the United States on December 11th, 1941, four days after the Pearl Harbor attack.
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Short and simple, primarily the attack on Pearl Harbor. And as a second reasoning was German U-Boats attacking American ships
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