MI5, Britain’s Security Service, tasked with counter-espionage and counter-terrorism, hit the headlines recently when their work in Northern Ireland was criticised. Public attention for MI5 is a relative rarity and undesirable, on the whole, so it’s perhaps surprising that they invited an independent historian to write an authorised history of their work to celebrate their centenary, and that it should be designed for wider publication. Having decided to do so, Professor Christopher Andrew, who has made the study of modern intelligence services his specialty, written accessible yet scholarly tomes on the KGB,  CIA and other services, and co-authored books with KGB defectors, was the obvious choice for the task.

In forewords to the main text, both the Director General of MI5 and Professor Andrew lay out the parameters of the exercise – what he could and couldn’t see, who he could and couldn’t talk to, what he could and couldn’t say. While he may have seen more classified material than he was actually allowed to write about, there is no doubt that Andrew has formed his own independent opinions about the successes and failures of the Security Service over the years. Of course, the failures are sometimes better known than the successes – the failure to detect the Cambridge Five, the Spycatcher affair, et al. Successes, often defined as they are by the absence of an event, tend to be less well known, especially post-war, and Andrew does his best to redress the balance there.

Starting at the very beginning, with the foundation of a counter-espionage bureau by Captain Vernon Kell a few years before the Great War, The Defence of the Realm takes us in comprehensive fashion through the operational adventures and political ups-and-downs of MI5: in both world wars they effectively rolled up the entire German espionage network in the UK, a remarkable feat made even more remarkable in the Second World War, when they turned a number of German agents (the Double-Cross System) and fed large volumes of mis-information back to the Nazis, making a huge contribution to Allied strategic deception. Success against the Russians was more elusive, and the difficulty of counteracting ideologically motivated high-flyers like Kim Philby and his cohorts was substantial – although the threat posed by Communism was the main focus for MI5 for the rest of the Cold War. The relationship between Whitehall and MI5 is explored, as is their monitoring of Communist influence within the Trade Union movement. Andrew dismisses the hoary old notion that MI5 somehow ousted Harold Wilson from Number Ten, and along the way counteracts most of Peter Wright’s other Spycatcher allegations, as well as visiting other key touchpoints in the history of the Service, including MI5′s growing role in Northern Ireland, and the shift from counter-subversion to counter-terrorism, which now consumes the bulk of their resources.

The Defence of the Realm is a fascinating book, not just as a history of the Security Service but because it gives an added dimension to a great deal of British political history (with Labour politicians making just as much use of MI5 to monitor the left wing as their Conservative counterparts). While there may be too much detail once in a while on departmental structures and the like, for the most part Andrew does a wonderful job of not just capturing the key work done by MI5, but also its social elements, its organisational character. Never afraid to draw his own conclusions, Professor Andrew has written a book that will remain definitive until the files on which it is based are opened up to later generations of historians – and we have a long time indeed to wait for that to happen.

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