MI5's performance in running a stable of double agents during the Second World War is still regarded as a textbook example of how such operations should be conducted.
The first agent to be recruited was the Welshman Arthur Owens who was arrested upon the outbreak of war because he was known to have been in contact with the German intelligence service. As the proprietor of a battery company he had often visited the Kiel shipyards, and had reported his observations to SIS. Unfortunately, as a mail intercept revealed, he was also in touch with the Abwehr, and this led to his detection.
While in custody Owens volunteered the fact that he had been entrusted with a wireless, and offered to make radio contact with the enemy under MI5's control. This unpromising beginning was to lead to a tremendous cryptographic breakthrough, because his hand ciphers were subsequently encrypted on the Abwehr's Enigma circuits, and he was also able to supply advance warning of the arrival in 1940 of several agents who were parachuted in Britain.
Two of these, codenamed 'Summer' and 'Tate', were to become important double agents. Indeed, 'Tate' was to continue his link with Hamburg from his arrival in September 1940 until the very last day of the war.
Through Owens, MI5 was able to acquire clues to the existence of other Abwehr spies, and provide suitably doctored documentation for future arrivals. Each individual was assigned a case officer from a specialist section within the counter-espionage branch to monitor every aspect of the agents' handling. A small inter-agency committee was created to liaise with other interested departments, especially to cope with the thorny issue of precisely what information could safely be conveyed to the enemy.
By January 1941 the system had been institutionalised under the supervision of the Double Cross Committee, under the chairmanship of an Oxford don, (Sir) John Masterman, and the number of double agents began to expand. Owens, who had outlived his usefulness and was considered unreliable, was incarcerated in prison, while MI5 established a secret detection centre at Ham Common in which suspects could be isolated and interrogated.
Garbo - Perhaps the most cunning of the SIS agents
The XX Committee, which came to be known as the 'twenty committee', because of the roman numerals, gradually expanded its activities to run some four dozen enemy agents, and eventually concluded that it had effectively taken control of the Abwehr's entire organisation in Britain. This was accomplished by skilful handling, the careful cross-referencing of information from agents with intercepted German wireless traffic which allowed MI5 an unprecedented insight into the enemy's intentions, and the development of a sophisticated conduit for conveying deception.
MI5's agents varied from volunteers, such as the legendary Spaniard codenamed 'Garbo', who invented two dozen notional sub-sources, the Yugoslav playboy 'Tricycle' who had worked previously for SIS, and 'Tate', who preferred collaboration to execution.
By 1944 MI5 was sufficiently confident of its double agents to be entrusted with 'Fortitude', the principal deception campaign intended to persuade the enemy that the widely expected Allied invasion of France would occur in the Pas-de-Calais. A secondary objective was to convey the impression that the landings in Normandy were merely a diversionary feint which could be safely ignored. The task of supplying this information rested with 'Garbo', who was highly regarded by the Abwehr, 'Brutus' a former Polish airforce officer who had been allowed to escape from German captivity in France, and 'Bronx', the daughter of a Peruvian diplomat who had travelled to Vichy for SIS.
'Fortitude' became a textbook example of how strategic deception should be conducted, and captured documents demonstrated that the enemy was duped not just about the date and place of the main Allied offensive, but even accepted that an entirely fictional army, the First United States Army Group, had assembled in south-east England in anticipation of crossing the Channel from Dover.
Victory for the double cross
By the end of the war the deception had been so complete that both 'Tate' and 'Garbo' had received German decorations in recognition of their espionage, and the entire operation had been financed by funds provided by the Abwehr.
Altogether an unlucky thirteen German agents were executed in Britain during the war, but more than three times that number had actively co-operated with their MI5 handlers. However, although Double Cross was widely heralded as an astonishing coup when details were declassified and released in 1974, Colonel T.A. Robertson, who had headed MI5's B1(a) section throughout the war, came to believe that some of his enemy counterparts must have realised their agents had been compromised, but chose to ignore the tell-tale clues.
Part of his evidence was an aborted experiment in which MI5 pretended to run a double agent so badly that the Abwehr would be bound to spot that he had come under MI5's control. In the event, the Germans continued to run 'Scruffy' and appeared to accept him as an authentic spy, so the case had to be abandoned.