Kevin Quinlan's new book, The Secret War between the Wars, has an impressive pedigree. It is the latest addition to the History of British Intelligence series edited by Cambridge's Peter Martland. Quinlan notes that the work “benefitted substantially” from his involvement with the University of Cambridge Intelligence Seminar, a select group presided over by MI5's past official historian, Christopher Andrew (xi). Andrew himself contributes a brief foreword, in which he praises Quinlan's effort as “path-breaking” and “powerful” (ix–x). On the whole, the book lives up to the promotion.

Quinlan holds a Ph.D. in history from Cambridge and currently lives and works in Washington, DC, where he is an analyst for the U.S. Department of Energy. In the past, he has served as a research analyst for the Royal United Services Institute and held positions with the U.S. Department of Commerce, USAID, and Goldman-Sachs. As he mentions in his acknowledgements, the book is a revised version of his dissertation. Fortunately, it is free of the excessive cautiousness and pedantic prose that often afflicts such projects. Overall, the style is straightforward, brisk, and thoroughly readable, if not exactly gripping.

This is a relatively brief volume. Of its total 286 pages, the main text takes up 185, organized into seven chapters. The remainder is made up of almost fifty pages of notes, plus bibliography, index, a foreword, a preface and an introduction, as well as three short appendices, notes on style, and a handy list of abbreviations and acronyms.

One of Quinlan's key points is that counterintelligence, at least as practiced by interwar MI5, was not dramatic, but rather a generally tedious, sometimes plodding, and frequently imperfect application of “tradecraft.” Quinlan broadly defines the term as “the particular methods an intelligence officer uses to operate and communicate with sources without being detected by the opposing intelligence service” (xx). This focus makes sense, as Quinlan is largely focused on “clandestine methods” involving human sources, or “HUMINT.” MI5's application (and occasional misapplication) of tradecraft is the heart of the book, a theme Quinlan explores through historical cases, including the 1926 General Strike, the 1927 Arcos Raid, Maxwell Knight's 1930s infiltration of pro-Axis circles, and the 1940 debriefing of Soviet defector Walter Krivitsky. As might be expected, the infamous Cambridge Five come up in the discussion, though they are not central to it. They and most of the cases in Secret Wars are more thoroughly discussed elsewhere, and anyone familiar with them is unlikely to learn much new about people and events themselves. Indeed, another work in the series, Victor Madeira's Britannia and the Bear: The Anglo-Russian Intelligence Wars, 1917–1929 (2014), covers much the same ground, albeit from a different angle.

What is unique to Quinlan's book is his detailing of how MI5 perceived and responded to various security threats. The historical cases noted are used to examine the application tradecraft in “official cover,” “recruitment and handling,” “penetration agents,” and “defection and debriefing.” Fittingly, the overwhelming source base for the book is the declassified MI5 files held in the National Archives at Kew. Quinlan supports this archive with a broad array of secondary sources, including, naturally, Andrew's. More problematically to some, perhaps, Quinlan also incorporates information obtained under Chatham House Rules, that is, anonymously, from “current and former government officials” (xi). However, this information is used very sparingly and is not critical to any of the book's arguments or conclusions.

In most of the cases examined in Secret War, the “other side” is represented by the Soviets, and, in Quinlan's analysis, success or failure often comes down to how well one side or the other practiced its tradecraft. Quinlan acknowledges Soviet intelligence as a formidable, though not infallible, opponent, but a basic limitation, not to say flaw, in his work is that he is really only looking at one side of what is a two-sided equation.

Overall, Quinlan presents the interwar MI5 as a highly professional organization that achieved important successes despite, as he frequently mentions, severe shortages of money and personnel. If there is a “hero” in the story, it is Maxwell Knight, the eccentric ex-Fascist who used female operatives to penetrate pro-Fascist and pro-Nazi cabals. His efforts culminated in the so-called Tyler Kent Affair, which involved a member of the American diplomatic community. However, Quinlan's characterization of Kent as a “bitter isolationist” is an oversimplification at best (128).

On the other hand, Quinlan is quite correct to argue that those who accuse MI5 and SIS of “gross incompetence” in their handling of defector Krivitsky by failing to heed his clues regarding MacLean, Philby, et al., are operating from 20–20 hindsight (175). As Quinlan demonstrates, Krivitsky's clues were really not that clear, and as an angry anti-Stalinist but still fiercely loyal Communist, he was a difficult source to handle and evaluate. Quinlan also correctly notes that the MI5 and Secret Intelligence Service files on the Cambridge Five remain, for whatever reason, classified, and the whole story can never known (if ever) until they are open to the public.

The Secret War between the Wars is a useful contribution to the study of interwar British intelligence, and interwar intelligence broadly, but it definitely will be most useful to readers who already have grounding in the period and the cases discussed. It offers an insightful look into the operational “mind” of MI5 during a critical and complex historical period.