What was the biggest mistake made during WWII?

[AdamL]

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The biggest mistake by leaps and bounds was the invasion of the Soviet Union. The war in the East was over before it had even begun.

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No, Germany would have won that one if they had mobilized their economy and planned for a 2 year campaign (at least). Instead they thought they could do it in one go, and that it would be like France. Nope, big difference.