Military and intelligence history mostly dealing with World War II.
I must say that I'm surprised that NSA still try to gain an upper hand over codemakers by builing another supercomputer. It has been widely assumed for decades that technology favours codemakers, not codebreakers. A thousandfold progress would mean that a brute force attack which formerly took the age of the universe (15 billion years) will "only" take 15 million years now. Meanwhile, the codemaker has simply go from 128-bit key to 138-bit to multiply the duration of a brute force attack by 1028. But well, maybe the US SIGINT' biggest secrets are in clandestine methods, work reduction factor or cyber intelligence.
Who knows maybe they discovered a flaw in AES or some new mathematical theorem.