The attack on Pearl Harbor by Japan in 1941 led to America's entry into World War II.
The event that led to America's entry into World War II was the attack on Pearl Harbor by Japan on December 7, 1941. Prior to this, the United States had been providing support to Britain but had not officially entered the war. The attack on Pearl Harbor changed that stance, prompting the U.S. to declare war on Japan the next day.
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