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So I think everyone here who has even the slightest knowledge of the second world war knows the importance to the allies of their successfull codebreaking of the German and Japanese codes.
So lets assume that for some reason the allies are not as successfull at breaking either the German or the Japanese codes. Either by the Germans and/or Japanese improving their encoding routines or by the allies not getting their hands on codebooks or encoding machines for instance.
What would the effects be?
I think that the naval war both in the Atlantic and definetly in the Pacific would be quite different.