Nearly thirty years ago, one trailblazing study of intelligence history cast intelligence as the ‘missing dimension’ of the history of international relations. Although privately many historians resisted the advent of intelligence history, that sentiment now appears as parochial as when Oxbridge academics did not teach history beyond the nineteenth century. Although some academics still privately dismiss intelligence history as lightweight scholarship, from the episodic furore surrounding the Cambridge Five revelations over the past fifty years and the parliamentary inquiry to the more recent publication of The Mitrokhin Archive (1999), the intense governmental concern with intelligence suggests that it is relevance rather than mere popularity driving the rapid expansion of the field as a focus of academic study. In the early twenty-first century, the importance of intelligence policy and the study of the methods, politics and ramifications of intelligence operations have all gained renewed importance in the wake of the 9/11 terrorist attacks and the wars in Afghanistan and Iraq. Earlier eras present similar issues now drawing scholarly attention.

Insider accounts and memoirs comprise most of the literature on intelligence published in the twentieth century. Despite the strict government policy regarding classification, they have been very revealing. Milestone studies include J. C. Masterman’s The Double-Cross System (1973), which recounted his work as head of the Twenty Committee (so named for the roman numerals representing ‘double-cross’, ‘XX’) and told of British success at doubling Abwehr agents during the Second World War, and Frederick Winterbotham’s The Ultra Secret (1974), which revealed the story of the British success breaking of German codes during the Second World War. The government itself opened further chinks in the armour of secrecy, first with the publication of Professor M. R. D. Foot’s officially-sanctioned SOE in France (1966) and then with the multi-volume official history British Intelligence in the Second World War (1979–1990) by Sir F. H. Hinsley et al. Many assumed that the continued classification of the intelligence services’ files kept it a closed subject, but Professor Christopher Andrew’s Secret Service (1985) showed that despite the continued classification of the intelligence archives, a rigorous, scholarly approach to intelligence history was not only possible through the investigation of alternative depositories but also necessary to understand twentieth-century international relations.