Double crossing the Nazis

Most people interested in the Second World War know of the Double Cross organisation – the brilliant initiative by British Intelligence in which all the German spies who entered Britain were turned into double agents and fed false information back to German Intelligence. The material they transmitted by wireless was instrumental in misleading the Nazi war machine and a crucial part of the success of the D-Day landings in Normandy.

The existence of the Double Cross organisation was revealed as long ago as 1972 by Colonel John Masterman, its wartime chief, in his book The Double Cross System in the War 1939-1945 – much to the annoyance of his former employers, MI5.

At that time much remained secret about the intelligence war, especially the part played by Bletchley Park in decrypting German Enigma traffic, so that Masterman was able to tell only half the story.

More recently, Ben Macintyre of The Times has gained access to the wartime files and has filled in the all-important details in his book Double Cross. And it is the details that are the most fascinating part of this deliciously twisting and tortuous story.

There was one detail in particular that jumped out of the pages at me and that was about the close connection and interdependence between the Double Cross committee and Bletchley Park.

Running a total of some twenty or so double agents for five years was a highly risky business. The gossip, chickenfeed, plus some apparently very accurate observations about military dispositions, was all carefully co-ordinated to add up in the mind of German Intelligence to a single theme: the Allies were planning to start a second front by attacking directly across the Channel at the Pas de Calais. Landings in Normandy were merely a preliminary diversion.

But if even one of the double agents turned out to be a treble agent – still secretly spying for the Nazis – or if one agent made a significant mistake, then German Intelligence would be alerted that their whole network was a fake and would read the intelligence sent in reverse. They would know for sure that Normandy was the target and Calais the bluff. Instead of fooling the Germans, British Intelligence would have accidentally handed them the true Allied plans on a plate.

Given the very high level of risk, it is amazing that the Allied military planners sanctioned the Double Cross system at all. But there was a second reason for keeping up the deception and this was probably the real reason for taking such risks. It was that the Enigma decrypts emerging daily from Bletchley Park gave a blow by blow account of what German military intelligence was thinking, what it accepted as real, and what it passed on to the Army High Command and to Hitler.

And although they remained on the same knife edge until the war’s end, officers of the Double Cross committee were reassured on a daily basis that the material they fed to the German was accepted and acted on. Equally important for the Allied war effort, the radio transmissions sent by agents like Tricycle, Garbo and Treasure, were trusted to such a degree that they were often re-sent, word for word in that day’s Enigma code, to German army headquarters.

This meant that the codebreakers at Bletchley had a ‘crib’ for that day’s Enigma settings, because they knew in advance what message German Intelligence would send. At the most crucial phase of the war in 1944, Bletchley Park was decrypting German intelligence messages and military orders almost as soon as they were sent – often reading the messages before the German High Command had deciphered them.

The deception worked so well that Hitler and the high command remained convinced, even six weeks after the Normandy landings, that the real invasion was yet to arrive in Calais.

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