Even the deception plan itself had a codename, Bodyguard, from Churchill's comment in November 1943 that truth was so precious 'it needed to be accompanied by a bodyguard of lies'.
The operation was run from a secret office in Battersea, south London, part of Churchill's war HQ. It was headed by Colonel John Bevan, but worked very closely with C, the head of MI6, with MI5 the domestic counter- intelligence organisation, and a branch of MI5 known as the XX committee, which ran turned German agents. By 1944 agents realised that Hitler was finished, and eventually 20 were recruited, and used to feed disinformation back to the Germans.
The aim was to convince the Germans that two alternative operations were taking place alongside the genuine one, which they tried to downplay as far as possible.
The first was a fictitious invasion of Norway, involving Sweden coming into the war on the side of the Allies and an invasion of Germany from the north though Denmark. They made it look as though an invasion army was gathering in Scotland but nothing could persuade the Germans that Norway was the target.
The second was a fictitious invasion of France through the shortest crossing over the straits of Dover to the Pas-de-Calais. A barrage of radio activity continued throughout and beyond D-Day to convince the Germans that the Normandy invasion was a feint.
Part of this strategy was to make the Germans believe that the invading force was larger than it was. If 20 divisions landed in Normandy, they could not be sure that another 20 divisions were not waiting to make a second assault later on the Pas-de-Calais.
The Germans were confused over the size of invasion force to expect. Rommel's intelligence predicted the Allies would land 35 divisions, but Hitler's staff were expecting 85 or 90. They were led to believe that Normandy was a diversionary tactic and delayed redeploying their defences for crucial weeks.
Dummy armies were established to convince the Germans that forces were gathering on the east coast of Britain as well as in Sussex, Dorset and the West Country. Fake landing craft were moored in the Thames and Medway estuaries, and replica gliders and inflatable rubber tanks were deployed across Kent and Essex.
Some of the false information fed back through turned agents by the XX (doublecross) committee of MI5, chaired by an Oxford don, John Masterman, was completely fabricated, including the existence of an American force in Iceland and a huge fake oil dock in Dover.
The American Third army was 'moved' from Cheshire to Chelmsford in Essex, where it was to be mistaken for the invasion army, a fictitious American First Army, commanded by the real General George Patton.
Part of the art of deception is to persuade the enemy of something they want to believe. The Germans were predisposed to accept that the invasion would be through the Pas-de-Calais.
They thought the Allies needed to capture a natural port, and the only ports in Normandy, Cherbourg and Le Havre, had been prepared for virtual demolition when necessary and were about 100 miles from England, allowing plenty of advance warning.
The secret invention of a floating harbour by the Allies meant Normandy was possible without storming a natural port. But to keep the secret, the men and women working on the Mulberry harbours had to be kept in ignorance of their task, each constructing a part but no one knew the shape of the whole.
The bad weather which hampered the invasion helped the deception. The German navy believed wind speeds of less than 30mph and waves of less than seven feet would be needed. This left them off their guard in the early June storms.
It is hard to say how successful the various strategems were. One exercise sent a double of General Montgomery on a 1,000-mile trip to Algiers and Gibraltar on 26 May, less than two weeks before D- Day. The Germans were supposed to think that if he was abroad, he could not be about to help lead the invasion.
Monty's double, a former actor Mayrick Clifton-James, learned to impersonate the General. He had a missing finger, which had to be covered that with a bandage to complete the disguise. His cover was never blown, crowds cheered at airports, but there is no evidence that the Germans noticed the trip at all.
On D-Day the German commanders did not know that the invasion was coming. Rommel had gone to Germany to celebrate his wife's birthday; Sepp Dietrich, commander of 1st SS Panzer Corps, was in Brussels; and Edgar Feuchtinger, commander of the 21st Panzer Division, was away with his lover. When news of the landing arrived at Hitler's headquarters at Berchtesgaden no one wanted to wake him from his drugged sleep.
Probably the success owed as much to weaknesses on the German side as to Allied brilliance. German intelligence departments were incompetent. Colonel Baron Alexis von Roenne, in charge of Fremde Heere West (FHW), the western intelligence arm, who had built his reputation on predicting Allied behaviour early in the war, was wrong on almost every important count other than to discount an invasion of Norway.
He was fooled by the fantasy invasion of the Pas-de-Calais and was convinced that the fictitious British Fourth Army in Scotland existed and was about to be redeployed to Kent.
Hitler himself, the German supreme command OKW (Oberkommando der Wehrmacht), and the commander Von Runstedt believed the intelligence reports. Hitler had an intuition as early as March that Normandy was a likely target, but only spread the defences thinner.
Certainly Rommel fortified the German defences along the entire 3,000 mile Atlantic Wall. Bunkers were constructed, positions dug, minefields laid, often in areas where the British had no intention of landing.
When the invasion came, a combination of confusion in the German high command and at very least the German failure to break British intelligence, left them relying on no more than guesswork, and all they had done was to hedge their bets about where and when the invasion would come.
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