Knowing an opponent’s intentions is among the most useful pieces of information to bring to any confrontation. But as history has shown with some painful regularity, getting it right can be brutally difficult. In deciphering the layer upon layer of signalling that occurs in international relations, penetration agents, those trained by to infiltrate a hostile organisation, provide one of the best means of distinguishing between the real and the counterfeit, the overt and the secret, and the secret and the mysterious.

The difference between secrets and mysteries is often critical: ‘Secrets are things that are potentially knowable’, whereas in mysteries, ‘there are no clearcut answers, often because the other leaders themselves do not know what they are going to do or have not worked out their problems’. The difficulty of assessing intentions, a complicated endeavour in the best of circumstances, is exacerbated when the adversary operates in a covert or clandestine manner, in secretive cells, and perhaps using legal political processes to shield illegal activity. MI5 experienced this problem when trying to counter the subversive activities of communists in the 1930s, as they had in the 1920s. British security and intelligence services knew full well that the broad goal of the USSR, through the CPGB, was to stoke the flames of rebellion in Britain, but the communists’ operations intended to effect sedition were not nearly as apparent. Security intelligence knew the what of communist ideology but not necessarily the how, when or who of Soviet operations.

Maxwell Knight, who gave his name to MI5’s ‘M. Section’ (M.S.), helped to fill part of the knowledge gap through the use of penetration agents. In an apparently rare discussion that directly addresses tradecraft, Knight made no secret of just how important he considered such agents. ‘The proper function of M.S.,’ he wrote, ‘lies in the recruitment and operation of agents who are trained for the purposes of penetrating subversive political bodies, and for the investigation of suspicious individuals or groups of individuals.’ These ideas about recruitment and agent-handling, seldom laid out with such candour, illuminate MI5’s agent operations against the CPGB at that time.