The Double Cross System or XX System, was a World War II anti-espionage and deception operation of the British military intelligence arm, MI5. Nazi agents in Britain were captured and used by the British to broadcast mainly disinformation to their Nazi controllers. Its operations were overseen by the Twenty Committee, under the chairmanship of John Cecil Masterman; the name of the committee comes from the number 20 in Roman numerals: "XX".

The policy of MI5 during the war was initially to use the system for counter-espionage. It was only later that its potential for deception purposes was realised. Agents from both of the German intelligence services, the Abwehr and Sicherheitsdienst (SD), were apprehended. Many of the agents who reached British shores turned themselves in to the authorities. Still others were apprehended when they made elementary mistakes during their operations. Later agents were instructed to contact agents in place who, unknown to the Abwehr, were already controlled by the British. The Abwehr and SD sent agents over by a number of means including parachute drops, submarine and travel via neutral countries. The latter route was most commonly used, with agents often impersonating refugees. After the war it was discovered that all the agents Germany sent to Britain had given themselves up or been captured, with the possible exception of one who committed suicide.

Methods of operation

The main form of communication that agents used with their handlers was secret writing. Letters were intercepted by the postal censorship authorities and some agents were caught by this method. Later in the war, wireless set were provided by the Germans. Eventually transmissions purporting to be from one double agent were facilitated by transferring the operation of the set to the main headquarters of MI5 itself. On the British side, a critical aid in the fight against the Abwehr and SD was the breaking of the German ciphers. Abwehr hand ciphers were cracked early in the war, and SD hand ciphers and Abwehr Enigma ciphers followed thereafter. The signals intelligence allowed an accurate assessment of whether the double agents were really trusted by the Germans and what effect their information had.

A crucial aspect of the system was the need for genuine information to be sent along with the deception material. This need caused problems on a regular basis early in the war, with those who controlled the release of information reluctant to provide even a small amount of relatively innocuous genuine material. Later in the war, as the system became a more coherent whole, genuine information was integrated into the deception system. For example, one of the agents sent genuine information about Operation Torch to the Germans. It was postmarked before the landing, but due to delays deliberately introduced by the British authorities the information did not reach the Germans until after the Allied troops were ashore. The information impressed the Germans as it appeared to date from before the attack, but it was militarily useless to them.

Operation outside the United Kingdom

It was not only in the United Kingdom that the system was operated. A number of agents connected with the system were run in Spain and Portugal. Some even had direct contact with the Germans in occupied Europe. One of the most famous of the agents who operated outside of the UK was Tricycle. There was even a case where an agent started running deception operations independently from Portugal using little more than guidebooks, maps and a very vivid imagination to convince his Abwehr handlers that he was spying in the UK. This agent, Garbo, created an entire network of phantom sub-agents and finally succeeded in convincing the British authorities that he could be useful. He and his phantom sub-agents were absorbed into the main Double Cross system, and he became so respected by the Abwehr that they stopped landing agents in Britain after 1942. They thus became wholly dependent on the spurious information which was fed to them by Garbo's network and the other Double Cross agents.

Results

Masterman expressed the opinion that as a consequence of Double Cross's efficacy, by early 1941, "we [MI5] actively ran and controlled the German espionage system in this country [United Kingdom]." This was confirmed after the end of the war.

Operation Fortitude and D-Day Landings

The British put their double agent network to work in support of Operation Fortitude, a plan to deceive the Germans about the location of the invasion of France. Allowing one of the double agents to claim to have stolen documents describing the closely guarded invasion plans might have aroused suspicion. Instead, agents were allowed to report minutiae such as insignia on soldiers' uniforms and unit markings on vehicles. The observations in the south-central areas largely gave accurate information about the units located there: the actual invasion forces. Reports from southwest England indicated few troop sightings, when in reality many units were housed there. Reports from the southeast depicted the real and the notional Operation Quicksilver forces. Any military planner would know that to mount a massive invasion of Europe from England, Allied units had to be staged around the country, with those that would land first nearest to the invasion point. German intelligence used the agent reports to construct an order of battle for the Allied forces that placed the center of gravity of the invasion force opposite Pas de Calais, the point on the French coast closest to England and therefore a likely invasion site. The deception was so effective that the Germans kept 15 reserve divisions near Calais even after the invasion had begun at Normandy, lest it prove to be a diversion from the main invasion at Calais.

The Allies were willing to risk exposing the Double Cross network to achieve the needed surprise for the Normandy invasion. However, early battle reports of insignia on Allied units that the German armies encountered only confirmed the information the double agents had sent, increasing the German's trust in their network. Some of the double agents were informed in radio messages from Germany after the invasion that they had been awarded the Iron Cross.

V-weapons deception

During the Blitz, the British noticed that V-1 flying bombs were falling 2–3 miles short of Trafalgar Square (the actual Luftwaffe aiming points such as Tower Bridge were unknown to the British). Duncan Sandys was told to get MI5-controlled German agents such as Zig Zag to report the V-1 impacts back to Germany. In order to make the Germans aim short, the British used the double agents to exaggerate the number of V-1s falling in the north and west of London and not to report, when possible, those in the south and east. For example, circa June 22, 1944, only one of seven impacts was reported as being south of the Thames when ¾ of the impacts had been there. Although Germany was able to plot a sample of V-1s which had radio transmitters, which confirmed that they had fallen short, the telemetry was disregarded in favor of the human intelligence.

When the German 65th Army Corps received a false Double Cross V-1 report that there was considerable damage in Southampton —which had not been a V-1 target—the V-1s were temporarily aimed at the South Coast Ports. As a result, the Double Cross deception also caused retargetting from London, not just inaccurate aiming. However, when V-1s launched from Heinkel He 111s at Southampton on July 7 were inaccurate, Frederick Lindemann, 1st Viscount Cherwell, recommended the agents report that the attack caused "heavy losses" in order to save hundreds of Londoners each week at the expense of only a few lives in the ports. When the Cabinet learned on August 15 of the deception, Herbert Morrison said that they had no right to decide that one man should die while another should survive, but the deception was approved to continue.

Moreover, when the subsequent V-2 rocket blitz began with only a few minutes from launch to impact, the deception was enhanced by providing actual locations (genuinely damaged by bombing) for impacts in central London, but each time-tagged with the time of an impact that had fallen 5–8 miles short of central London. From mid-January to mid-February 1945, the mean point of V-2 impacts edged eastward at the rate of a couple miles a week, with more and more V-2s falling short of central London.

List of Double Cross agents

• Brutus
• Charlie
• Garbo
• GW
• Mutt and Jeff
• Rainbow

• Snow
• Tate
• Treasure
• Tricycle
• Zig Zag

• Beetle
• Dragonfly
• Summer
• Father
• Fido
• Careless

Notes

1. NOTE: Ordway/Sharpe cite Masterman

References

• Hinsley, F. H., and C. A. G. Simpkins. British Intelligence in the Second World War, Volume 4, Security and Counter-Intelligence. London: H.M. Stationery Office, 1990. ISBN 0-11-630952-0.
• Howard, Michael British Intelligence in the Second World War, Volume 5, Strategic Deception London: H.M. Stationery Office, 1990. ISBN 0-11-630954-7
• John C. Campbell, "A Retrospective on John Masterman's The Double-Cross System", International Journal of Intelligence and CounterIntelligence 18: 320–353, 2005.
• Jon Latimer, Deception in War, London: John Murray, 2001
• Public Record Office Secret History Files, Camp 020: MI5 and the Nazi Spies, Public Record Office, 2000