A major claim of the recent group of "intelligence historians" is that the study of the secret world is the "missing element" in contemporary history - that, just as the history of the second world war needed revising after the British government finally let on about ULTRA, history (especially of the Cold War) is missing the perspective provided by intelligence. Richard Aldrich's GCHQ is certainly part of this project, just as his The Hidden Hand was one of the better works in it for covert action, propaganda, and human intelligence.
But do we know that much more about the main line of history from it? There are, of course, a couple of serious documentary and methodological problems with this. Even where we do have good sources on the history of secret intelligence, it's typical for the actual intelligence product to remain secret. We have a reasonable idea of what all those antenna farms were after - we don't have much, post-ULTRA, of what the prime minister actually got delivered to his desk in the blue-jacketed files. Writing my own Master's thesis, I remember that the literature was rather better on the contribution of Soviet intelligence to the 1973 crisis than the US kind, but even that was because various individuals had been forthcoming. The Soviets tried to persuade Sadat to end the war by producing MiG-25R imagery showing the Israeli counter-offensive building up; he wasn't apparently convinced. We don't know, however, if the Americans did anything similar with the Israelis, although we do know that the Israelis weren't sharing their own information with the Americans. (And we know now that Ted Heath turned off their SR-71 operation out of Lakenheath, so how much did they know?)
There's another problem, though, which is understanding what contribution intelligence actually makes to decisions. Cynically, you might say that giving politicians more data is pointless; they'll either ignore it or pick the bits that suit their preconceptions. John Keegan argued that across history, intelligence was more often misused, ignored, or just irrelevant to the balance of forces on the ground than not. Obviously, having regular deliveries of ULTRA decrypts didn't prevent Dunkirk, although it may have helped bring off the evacuation. Even more obviously, whatever intelligence sources Tony Blair was using in 2002 didn't bring him very much enlightenment. That raises another question - was the intelligence valid even before the upsexers got at it? Why did all the European countries with their own overhead imagery choose to stay out?
These problems are less serious when the events in question were motivated by intelligence interests, rather than by the content of intelligence. Aldrich is good on this - the times when "the SIGINT tail started to wag the policy dog". Notably, this seems to have been a major motivation in the whole sorry story of Diego Garcia, intended as a replacement for the abandoned sites on Mauritius and Ceylon and for the NSA's intelligence-gathering ships after the attack on USS Liberty. Around this time, GCHQ also considered building an enormous, nuclear-powered ship intended to contain a complete overseas station of the size of HMS Anderson on Ceylon or Little Sai Wan in Hong Kong, plus a BBC World Service transmitter site - Harland & Wolff's was commissioned to carry out a design study.
The plan was to have it flagged as a merchantman, but it would have been an enormous and expensive sitting duck.As plans go, at least it didn't involve ethnic cleansing.
Later, when the third Wilson government decided to pull out of the remaining overseas bases in 1976, it was the GCHQ interest, backed up by the NSA, that led them to keep the presence on Cyprus - as well as huge British intelligence facilities, the Americans had transferred numerous organisations there from Turkey when the Turks asked them to leave, which had then moved into the British bases for security after the 1974 invasion.