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The Allies entered the war on September 3, 1939, when Germany invaded Poland, which was allies with Britain and France.

The United States entered the war on December 7, 1941, when Japan, Hitler's ally, attacked pearl harbor, before declaring war on the United States, which they had intended to do minutes before the attack via a note from the Japanese representatives who had been in America to discuss peace terms, but the note was not delivered on time, but instead some time after the attack.

This is a very brief description of the reason for the allies entering World War 2, but is meant to be an overview. If you want to know more, I suggest looking up operations Barbarosa and Overlord on wikipedia.

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13y ago

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