This chapter examines how the Combined Services Detailed Interrogation Centre (CSDIC) in Britain developed its sophisticated system of interrogating German soldiers. While CSDIC’s use of microphones is now widely known, the process that produced its intelligence remain poorly understood. The chapter shows that CSDIC was initially designed to gather pieces of information overheard among soldiers, but eavesdropping alone soon proved insufficient for the scale and complexity of the war. Soon enough, CSDIC UK had become the final step in a much larger screening process that sifted hundreds of thousands of soldiers to identify a select group for the bugged sites. The actual process in its camps then formed a cycle: statements obtained in direct interrogation were checked against private conversations between prisoners, which in turn revealed new leads for direct interrogation. Microphones, therefore, were part of a broader interrogation method rather than its entirety. The chapter also highlights the people at the centre of this system, whose work yielded a remarkable body of intelligence, yet its emotional and psychological toll remained largely concealed by secrecy well after the war.

CSDIC is best known for the transcripts of prisoner conversations it generated through covert listening. Yet these dialogues stand largely on their own: they contain no material to explain why particular prisoners and conversations were monitored. The chapter argues that to understand the purpose of these transcripts, we need to look beyond them, to the other kinds of reports CSDIC produced that were not derived from microphones. Numbering roughly 2,500 documents, these reports chart a transformation in the information Britain sought to extract from these prisoners. The chapter shows how CSDIC was initially concerned with military information, but that it soon expanded in both reach and ambition, moving from tactical and operational intelligence to more specialised studies aimed at understanding enemy morale and shaping post-war occupation planning. Alongside these efforts, its microphones were also capturing references to atrocities and a growing awareness among some soldiers of the genocidal intent behind them. Yet such revelations remained subordinate to institutional priorities about protecting CSDIC’s methods even after 1945.

What emerges from the three cases is that interrogation was a central component of the wartime state, and that it remained essential immediately after the war. It also delivered: each camp extracted the information it was tasked to obtain, and its output was generally considered reliable. At the same time, there were biases that ran so deep that they were not recognised, with the three cases confirming Britain’s failure to grasp the extent and nature of mass murder and genocide, even as told by its own sources. The comparison also shows that interrogation relied on forms of psychological manipulation, though the character and intensity of those methods differed between the different camps. At the same time, higher command had an interest in ensuring that all detainees were treated in a manner consistent with Britain’s self-image and legal obligations. While abusive practices did occur, they affected a relatively small number of those under interrogation and were out of sync with standard practices. Taken together, these findings confirm that interrogation was not an isolated activity but one shaped by wider institutional priorities and cultural attitudes.

Interrogating Nazism uncovers a history in which interrogation became a valuable information-gathering method in the context of the Second World War, giving Britain remarkable knowledge of Nazi Germany in the process. Much of that knowledge, concerning individuals, armies, atrocities and numerous other subjects, could not have been gained through any other means in such detail. Not only was it obtained in interrogation camps, it was also preserved in tens of thousands of reports, indexes and verbatim transcripts of secretly recorded discussions. In many ways, the sources emanating from interrogation are a ‘first oral history’ of the Second World War. This is a term that David Cesarani used to describe the interviews of Allied psychologists with German soldiers – some of which had their origins in the interrogation camps studied here – but which is appropriate for much of the archival material that makes up this book.

British policy restricted investigations to incidents involving Allied civilians or prisoners of war, excluding the persecution of Jews and other targeted groups. This was a narrow and ultimately inadequate framework, but the investigations into war crimes themselves were far from parochial. They addressed episodes of violence that reflected the broader patterns of cruelty that defined the war. This chapter uses WCIU’s interrogation reports to examine the distinctive challenges the interrogations of so-called ‘minor’ war criminals posed. By following interrogators’ work on the ground, the chapter highlights the complexity of this effort that other records tend to omit. Unlike high-profile defendants such as Göring or Ribbentrop, these perpetrators were difficult even to identify, and their crimes were rarely documented. Investigators therefore relied on a combination of past intelligence – much of it collected through interrogation during the war – as well as on witness testimony. These investigations were as a result shaped by fraught encounters among suspects, witnesses and interrogators.