Just started reading a history of the NSA [1], and it's very interesting. E.g., the execution of the Rosenbergs was a travesty of due process, but the US had secretly decrypted a message listing them as Soviet spies. Another example, a midlevel employee of US SIGINT efforts told the KGB that the US was capturing and decrypting basically all Russian communication, and the Russian response was to harden their protocols (far less radio traffic, much greater use of one-time pads, basically.) The traitor was never prosecuted, because reporting the evidence against him would have revealed a SIGINT source.

Cryptology became the poor stepchild of US SIGINT efforts after this, because it yielded so little useful intelligence regarding Russia. As a result, the North Korean invasion which started the Korean war took the US completely by surprise. The US was subsequently able to crush the invasion in part because the North Koreans transmitted all communications in the clear, so they often knew what NK military units were going to do before they knew themselves. It took the US about a month to spin this capability up, though, because non-Russian communications had been completely neglected.

This neglect burned the US again when China decided to enter the war. The US could not believe this was happening, despite mounting SIGINT evidence, so China again had the advantage of surprise. China actually was encrypting its military communications in a way that the US could not decrypt (at least initially; I haven't finished reading this section of the book) which gave them a huge advantage in the initial engagements with US forces.

[1] http://books.google.com/books?id=x_K2rb-OShMC&printsec=f... (Currently only $3 for the e-book!)