After Soviet intelligence officer Walter Krivitsky defected to the West in 1937, he offered his receivers tremendous amounts of information and great insight into one of modern Europe’s most closed societies, one whose clandestine activities often left rivals ignorant and weak in the face of its subversive onslaught. Krivitsky’s debriefing in London, conducted over the course of four weeks in January and February 1940, occupies a singular place in the annals of MI5. The debriefing was the first of its kind, the earliest attempt at what would later become a standard practice in handling defecting or active enemy agents. It was MI5’s first experience interrogating a former Soviet intelligence officer, its first opportunity to directly question a former illegal about the machinery and tradecraft of Soviet intelligence. The novelty of Krivitsky’s debriefing does not alone, however, account for its prominence in MI5’s history. The seamless nature of the debriefing’s planning and execution, the expertise and diligence of the officers who conducted it, and the quality and quantity of the information it produced have led some MI5 insiders to regard this case as the moment when MI5 came of age.

A comparison of Krivitsky’s experiences in Britain and the United States, to which he had emigrated in 1938, contrasts the differences in security intelligence between the two nations, dramatically highlighting MI5’s relative sophistication at the time. Even given that his arrival in America occurred under very different circumstances than his arrival in Britain, disorganisation and a degree of incompetence or inexperience, at least as regards this case, are likely explanations for Krivitsky’s encountering a hostile US immigration service, parochial government officials and small-minded bureaucrats. His potential value as a high-ranking former intelligence operative was lost on the FBI until after his death. Krivitsky’s experience in America also revealed a nascent conflict between law-enforcement and intelligence gathering in the United States.

Although Britain’s intelligence services are by no means strangers to power struggles, MI5 and SIS both grasped the importance of Krivitsky’s defection and worked closely to ensure he was brought to London for the extensive and landmark debriefing, an event recognised as ground-breaking even at the time.