Intelligence remains the only profession in which a fictional character (James Bond) remains many times better known than any real practitioner, alive or dead. The tradecraft used by intelligence agencies tends to be thought of as the province of Bond and the spy novelist rather than the serious scholar. References to ‘invisible ink’, for example, are vanishingly rare in the many scholarly studies of twentieth-century international relations published by university presses. It was not always thus. Robert Boyle of ‘Boyle’s Law’, one of the key figures in the seventeenth-century scientific revolution and a founder of the Royal Society, invented the term ‘invisible ink’ and devoted much research to the way reagents make messages written in it ‘confess their secrets’. As Professor Kristie Makrakis has argued, the fact that until 2012 the CIA refused to declassify files on the use of invisible ink in the First World War (the oldest classified documents in the US National Archives) is evidence of their continued relevance to intelligence tradecraft. The simplest methods of transmitting secrets are still sometimes the best.

As Kevin Quinlan argues persuasively, ‘The most successful intelligence gathering has depended upon good tradecraft.’ His path-breaking (and often colourful) analysis of the interwar development of MI5 tradecraft opens up new and important perspectives on counter-espionage, counter-subversion and the British response to Soviet Communism, Italian Fascism and German Nazism. The Secret War between the Wars also has major implications for our understanding of the role of intelligence in both the Second World War and the Cold War. MI5 tradecraft was essential to the success of probably the most effective strategic deception in the history of warfare: the ‘Double-Cross System’ which fed Hitler and his high command with an unprecedented volume of high-grade disinformation.

Soviet tradecraft also had major successes. The sophistication of the intelligence tradecraft deployed against Moscow’s wartime allies enabled it to capture probably the single biggest secret in the history of espionage: the plans of the first US atomic bomb, on which the Soviet Union modelled its own. Superior tradecraft also allowed Soviet intelligence to profit from the sometimes woeful lack of security in Western embassies in Moscow. When an FBI expert carried out the first, long overdue, electronic sweep of the US embassy in 1944, he found 120 hidden microphones in the first 24 hours.