Being able to read diplomatic messages is a definite gold-mine. Of course, knowing the contents of diplomatic messages isn't always enough. A good example is

Being able to read diplomatic messages is a definite gold-mine. Of course, knowing the contents of diplomatic messages isn't always enough. A good example is described in Peter Wright's Spycatcher: the Brits were breaking the French diplomatic cipher, using an ingenuous attack on the electromagnetic noise of the cipher machine in the embassy. But all this intelligence was unable to stop De Gaulle thwarting their entering the European Common Market.