What was the biggest mistake made during WWII?

[Andrew Karpinski]

Put yourself in either the position of the axis or the allies. What do you feel was the biggest mistake made, on either side, during the course of the war? The scope of this question is intentionally broad.

My entry for the axis is the German decision to invade Russia. If they were able to focus simply on the battle of Britian it is quite possible that the war could have been won. Fighting a dual front battle made victory nearly impossible.

On the side of the allies I think the french's inability to adapt to modern warfare and inattention to Charles Degaules theories takes the cake.