What was the biggest mistake made during WWII?

[The once and future king]

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The Soviets might have done a better job tactically had not Uncle Joe killed or exiled to Siberia much of the top echelon of the Red Army in '37.

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Top Echolon staff have more to do with things at the strategic or operational level. Whilst the soviets where piss poor on the whole in 41 at this level later in the War they totaly pwned the Germans operationaly.