I'd shared the view of most historians that there wasn't material for a book but one could write about particular episodes. In looking at what other people had written on the subject I realised that they'd all made what seemed to me to be an error. Having done that I realised I could write more. The discovery that I could write the complete history was a progressive revelation. Secondly professional historians had been put off by the James Bond idea that intelligence consists of one dazzling coup which changes the course of history.
People think of cloaks and daggers and dramatic coups but all that intelligence is, is information.
Once they've got it, it can be even more difficult to use it. Indeed, the book reveals a startling series of blunders in the use of intelligence. The modern Secret Service has its basis in the notion that Britain was teeming with German spies in the run up to World War One. An idea that was later shown to be quite erroneous. The existence of Ultra remained secret up until 1975 when it was de-classified.