The Double-Cross System

"We actively ran and controlled the German espionage system in this country." John Masterman, an Oxford historian who chaired the committee in charge of Britain's double agents, wrote that in 1945. MI5 still quotes it as the verdict on what it calls its main success of the Second World War^1,2.

SNOW and the first lessons

The system began with one awkward agent. Before the war MI5 learned, partly through a former fighter pilot it was running against German intelligence, about a Welsh-born electrical engineer in contact with the Germans. It code-named him SNOW. Shortly after war broke out in September 1939, SNOW became the first German agent MI5 turned¹.

SNOW, whose real name was Arthur Owens, proved hard to read. In early 1941 MI5 sent another agent, Walter Dicketts (CELERY), to Lisbon to meet SNOW's German controller. Each man came back throwing doubt on the other. MI5 could not settle who was lying, interned Owens, and shut down the whole network built around him. The case taught MI5 how the Abwehr handled its agents⁶.

Catching and turning

From mid-1940 the Abwehr sent agents to Britain by parachute, by boat and through neutral countries posing as refugees. Fewer than 25 arrived between September and November 1940. Most were poorly trained, and MI5 picked up almost all of them⁶.

Two things made this possible. Before the war MI5 had studied how German intelligence kept in touch with its agents. During the war, codebreakers at Bletchley Park (GC&CS) read Abwehr messages, first its hand ciphers and later its Enigma traffic. The decrypts often gave warning of an agent's arrival, and later showed whether the Germans still trusted the agents MI5 was running^1,6.

Captured agents went to Camp 020 at Latchmere House in Richmond for interrogation under Lieutenant Colonel Robin Stephens. Those judged usable were offered the chance to become double agents. MI5's section B1A, under T. A. Robertson, ran the cases^6,2.

The agents sent their reports mainly by secret writing in letters, by radio, and in some cases by microphotography or in person in neutral Lisbon and Madrid^2,6. Every report needed enough true information to keep the Germans' confidence. Before the 1942 landings in North Africa, one of GARBO's notional sub-agents reported a convoy leaving the Clyde in Mediterranean camouflage. The letter was postmarked before the landings but arranged to reach the Germans too late to be of use. They thanked him anyway³.

The Twenty Committee

From January 1941 the information the double agents passed was coordinated by the Twenty Committee, named for the Roman numeral XX, a double cross¹. Masterman chaired it. Its members came from the service intelligence branches, MI5 and MI6, and it decided what each agent could tell the Germans^7,1. The committee's secretary, Charles Cholmondeley, later co-devised Operation Mincemeat⁷.

The cast

MI5 records about 120 double agents over the war¹. A few illustrate the range.

• MUTT and JEFF, two Norwegians, came ashore in northeast Scotland in April 1941 and gave themselves up. Their radios told the Germans of an invasion threat to Norway, and MI5 staged a real explosion at a power station in Bury St Edmunds to support their claimed sabotage. The Germans made four parachute drops of supplies to them².
• TATE, the Dane Wulf Schmidt, parachuted in during September 1940. He stayed in radio contact with Germany until May 1945⁶.
• TRICYCLE, the Yugoslav Duško Popov, ran his own sub-network, including BALLOON and GELATINE. One MI5 file remarks on how alarmingly accurate BALLOON's information was².
• GARBO, the Catalan Juan Pujol García, had invented a network of agents in Britain for the Germans before MI5 took him on in 1942. With his case officer Tomás Harris he built it up to 27 imaginary sub-agents^3,1.

From counter-espionage to deception

At first MI5 used the system mainly to catch spies and to learn what the Germans wanted to know. Its value for deception came later^2,6. By 1944 the agents were trusted enough to carry the central story of Operation Fortitude, that the Normandy landings were a feint before a larger attack on the Pas-de-Calais. Berlin awarded GARBO the Iron Cross in July 1944^1,3.

MI5 also credits the system with successes in U-boat and V-weapon deception². During the V-1 attacks of 1944, agents including TATE and ZIGZAG were used to overstate hits in north and west London and understate hits to the south and east. The aim was to make the German launch crews shorten their range⁶.

Writing it down, and letting it out

Masterman wrote his report on B1A's double agents in 1945; it is now the MI5 file KV 4/5⁴. For more than twenty-five years it stayed secret. A Prime Minister's Office file covering 1971 and 1972 deals with the "question of publication," and the book appeared in 1972⁵. MI5 released the case files of individual agents later: Harris's GARBO files in January 1999, and a further group, including MUTT, JEFF and BALLOON, in November 2002^3,2.

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January – September 1944 Operation Fortitude The Allied deception that invented armies in Scotland and southeast England in 1944 to make Germany expect landings in Norway and the Pas-de-Calais, and to see Normandy as a feint. 4 February – 14 May 1943 Operation Mincemeat In 1943 British intelligence floated a dead man dressed as a Royal Marines officer ashore in Spain with forged letters pointing at Greece and Sardinia, to hide the coming invasion of Sicily. 1912–1988 Juan Pujol García (GARBO) A Catalan former poultry farmer who invented a spy network for the Abwehr, became MI5's double agent GARBO, and helped persuade Hitler that Normandy was a feint. 1912–1981 Duško Popov (TRICYCLE) Yugoslav lawyer recruited by the Abwehr in 1940 who became MI5's double agent TRICYCLE, carried a German questionnaire about Pearl Harbor to the FBI, and fed D-Day deception. TRADECRAFT 1920s–1990s Double agents Agents who seem to serve one service while controlled by another. From the OGPU's Trust to MI5's Double Cross system and the FBI's Duquesne case, the best of them turned an enemy network into a channel for deception. Germany · 1920–1944 Abwehr The German armed forces' intelligence service from 1920 to 1944: spies, saboteurs and counterintelligence under Admiral Canaris, outplayed by the FBI and MI5 and finally absorbed by the SS.