This article explores the development of signals intelligence within the trenches of the Western Front during the First World War. It explains how, using listening sets, the British belatedly imitated their ally and enemy by intercepting messages that had leaked from telephone lines or were transmitted through the ground. The rise and decline of this system is unpacked comprehensively and a fresh assessment is made regarding its military contribution. It concludes that previous interpretations are broadly correct, but they have underestimated the system’s influence and longevity. The article also clarifies how listening sets were used for eavesdropping on prisoners.
- Signals intelligence; communications security; prisoners; First World War; Western Front; British Expeditionary Force