In my opinion, the importance of the intelligence war in World War 2 cannot be overstated. Economically, Britain was essentially bankrupt by the end of the war. Militarily they were not as well resourced and equipped as Germany with the exception of naval warfare, where the odds were much closer than in World War 1. The most decisive factor in Britain being on the victorious side of WW2 was their superior intelligence capabilities. There is nothing new in the technique of turning the agents of the enemy into channels for deception and misdirection, but the British in WW2 turned it into an art form. The Double-Cross System by John Cecil Masterman describes perhaps the most effective example of the use of double agents in modern military history. (Read more...)
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