An accomplished Whitehall in-fighter, Capel-Dunn over-reached himself when he attempted to take control of the Security Service (MI5). Appearing unannounced at their headquarters on 24 October 1944, he claimed to be acting on behalf of the JIC in an investigation into all sources of intelligence and their distribution. When asked by the Director-General for his credentials he was unable to produce them. Threatened with an enquiry of the Minister, Capel-Dunn withdrew and no more was heard of the investigation. However he achieved a revenge of sorts when, at war's end, he published (for classified circulation only) an assessment of intelligence operations.