What was the biggest mistake made during WWII?

[ChipWrecked]

Hiroshima and Nagasaki were legitimate military production centers, set in valleys ringed by mountains that helped reflect the shock back onto the target. The targets were carefully chosen. I have no problem there.

Kyoto was specifically spared bombing by the Allies because of its cultural significance.

Western Allies: appeasing Stalin by agreeing to the division of Europe, thus setting the stage for WWIII, the Cold War. What leverage did Uncle Joe have if Roosevelt and Churchill had told him to go [censored] in his hat?

Axis: Pretty much what Once and Future King said, and I never thought I would ever agree with him. Had Hitler concentrated on bombing the English production centers in the North, rather than terrifying civilians in London, he might have been able to take the British Isles depriving the Allies of their invasion base.