How a double agent works

1. The enemy’s agent

   The starting point is an agent of a foreign intelligence service. MI5, for example, uncovered a Welsh-born electrical engineer spying for the Germans and gave him the codename SNOW. Sometimes a person walks into an official building of their own accord (walk-in) or is deliberately put forward to be recruited (dangle).

2. Turning

   The CIA glossary describes a double agent as someone who first works for one service and then volunteers for, or is recruited by, a second – usually to feed the first false information or to spy on it. SNOW was the first of 120 wartime German agents whom MI5 turned into double agents.

3. The handler

   The agent is run by a case officer; the CIA calls this person a handler. Juan Pujol García, later GARBO, approached the British three times in 1941 without success and only then was taken on by German intelligence. By April 1942 he had reached MI6 and was brought to London, where his case went to the Spanish-speaking Tomás Harris.

4. Building credibility

   The enemy has to trust the agent. That is what chicken feed is for: genuine but not seriously damaging information passed on knowingly through the agent. GARBO convinced the Germans that he ran a network of 28 productive sub-agents (MI5’s page on the agent says at least 27). If the game comes to light, the agent is blown.

5. Keeping in touch

   The agent has to report regularly to the handlers on the other side. According to MI5, Pujol and Harris wrote 315 letters averaging 2,000 words each, and from January 1944 to D-Day more than 500 radio messages – four a day – passed between GARBO and Madrid.

6. Codebreakers’ support

   According to MI5, the codebreakers at Bletchley Park, who read German communications (Ultra), gave the whole system indispensable support.

7. Coordination

   From January 1941 the disinformation passed on by the double agents was coordinated by the Twenty Committee. The name comes from the Roman numeral XX, a ‘double cross’ – hence the Double Cross System.

8. The game

   The biggest stake was Operation Fortitude. GARBO helped persuade the Germans that the main landing in June 1944 would come around Calais rather than in Normandy. On 9 June he reported that the Normandy landings were only a diversion. It is a textbook case of deception.

9. The result

   According to MI5, the Germans kept two armoured divisions and 19 infantry divisions in the Pas-de-Calais through July and August 1944, which gave the Allies time to secure their bridgehead. On 29 July 1944 GARBO was told he had been awarded the Iron Cross, and in December 1944 he received the British MBE.

↺ The game lasts as long as the enemy believes the agent, and each reaction suggests the next move.