Double - Cross System 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 – real and false – were captured , turned themselves in or simply announced themselves and were then 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 " ( i . e . double crosses ) . 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 . In addition , some were false agents who had tricked the Germans into believing they would spy for them if they helped them reach England ( e . g . Treasure , Fido ) . 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 last 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 had been captured with the possible exception of one who committed suicide . Early agents . Following a July 1940 conference in Kiel , the Abwehr ( German intelligence ) launched an espionage campaign against Britain involving both intelligence gathering and sabotage . The spies were sent over from Europe in various ways ; some parachuted or came off a submarine . Others entered the country on false passports , or posing as refugees . Public perception in Britain at that time was that the country was full of well trained German spies who were deeply integrated into society . There was widespread , as Churchill put it , " spy - mania " . The truth was that between September and November 1940 fewer than twenty five agents arrived in the country ; mostly of Eastern European extraction , badly trained and poorly motivated . The agents were not difficult to spot - a task made still easier by the cracking of the German ' s Enigma encryption . MI5 , with advance warning of infiltration , had no trouble picking up almost all of the spies sent to the country . Writing in 1972 , John C . Masterman ( who would later head the Twenty Committee ) said that by 1941 MI5 " actively ran and controlled the German espionage system in [ the United Kingdom ] . " It was not an idle boast ; post - war records confirmed that none of the Abwehr agents , bar one that committed suicide , went unnoticed . Once caught , the spies were deposited in the care of Captain Robert Stephens at Camp 020 ( Latchmere House , Richmond ) . After Stephens , a notorious and brilliant interrogator , had picked apart their life history , the agents were either spirited away ( to be imprisoned or executed ) or , if judged acceptable , offered the chance to turn double on the Germans . Control of these new double agents fell to Thomas Argyll Robertson ( usually called Tar , from his initials ) , a charismatic MI5 agent . A Scot , and something of a playboy , Robertson had some early experience with double agents ; just prior to the war he had been case officer to Arthur Owens ( code name Snow ) . Owens was an oddity , and it became apparent that he was simply playing the Germans and British against each other – to what end Robertson was unable to uncover . The experiment had not appeared to be a success , but MI5 had learned key lessons about how the Abwehr operated and how double agents might be useful . Robertson , in particular , believed that turning German spies against their masters would have numerous benefits ; for example determining what information the Abwehr wanted or to actively mislead them as part of a military deception . In addition , it would discourage them from sending more agents if they believed an operational network existed . Section B1A ( a subordinate of B section , under Guy Liddell ) was formed and Robertson was put in charge of handling the double - agent program . Robertson ' s first agents were not a success ; Giraffe ( George Graf ) was never really used and Gander ( Kurt Goose ; MI5 had a thing for amusingly relevant code names ) had been sent to Britain with a radio that could only transmit , not receive . Both were quickly decommissioned . The next two attempts involved even more farce ; Gösta Caroli and Wulf Schmidt ( a Danish citizen ) landed , via parachute , in September 1940 . The two were genuine Nazis , had trained together and were friends . Caroli was coerced into turning double in return for Schmidt ' s life being spared , whilst Schmidt was told that Caroli had sold him out and in anger swapped sides . Caroli quickly became a problem ; he attempted to strangle his MI5 handler before making an escape carrying a canoe , on a motorcycle . He vaguely planned to row to Holland , but came unstuck after falling off the bike in front of a policeman . He was eventually recaptured and judged too much trouble to be used . Schmidt was more of a success . Codenamed ' Tate ' , he continued to contact Germany until May 1945 . However , these eccentric spies made Robertson aware that handling double agents was going to be a difficult task . 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 sets 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 cipher s 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 neutral 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 Dusan Popov ( " 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 , Juan Pujol ( " 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 fictitious network 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 . 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 centre 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 Germans ' 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 . The British noticed that , during the V - 1 flying bomb attacks of 1944 , the weapons 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 and TATE 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 favour 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 . V - 1s were extremely powerful . As a result , the Double Cross deception also caused " retargetting " from London , not just inaccurate aiming . However , when V - 1s launched from Heinkel He 111 s at Southampton on July 7 were inaccurate , British advisor Frederick Lindemann 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 locations genuinely damaged by bombing , verifiable by aerial reconnaissance , for impacts in " central London " , but each " time - tagged " with the time of an earlier 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 of miles a week , with more and more V - 2s falling short of central London .