The U.S. entered World War II after Japan's attack on Pearl Harbor in 1941, ending its isolationist policies.
The United States entered World War II following Japan's attack on Pearl Harbor in 1941. The attack served as a catalyst that propelled the U.S. into the war despite previous isolationist sentiments. As tensions had been escalating, with the U.S. supporting Britain against Germany and longstanding issues with Japan over power in the Pacific.
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