Because they had totally different beliefs. It was either destroy the 'different poeple' (Jews, Homosexuals,homeless,etc.) or let the people go free. Which one would you think? The side you disagree with were perpared to kill you, child or not, they would not care.
Edit: You're smattering. There are much more other reasons. I'm really scared about the naivete of the most people in wiki answers ...
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Hitler's Germany, Mussolini's Italy and Tojo's Japan were on the Axis powers.
Enemy of who? The Allied or Axis Powers?
Allied Powers only did it to surprise the western part of Europe, and the Axis Powers.
It would keep the allied powers safe and help them defeat the axis powers
the allied powers were the United States, Britain, France, Russia And the axis powers were mainly Germany, Italy, and Japan