One reason for the success of the Allied invasion of Europe on D-Day 70 years ago was that the Nazis were not expecting it. Or at least not when and where it happened. That’s thanks to a massive, years-long deception campaign.

“The element of surprise was absolutely vital,” says Nicholas Rankin, author of Churchill's Wizards and other books about intelligence and deception in World War II. “Montgomery, Eisenhower and all the generals paid tribute to this,” he adds.

“It was years of work that went into this particular deception,” Rankin says. “And the deception was to make the Germans think that the invasion was going to happen at Calais, rather than in Normandy.”

Americans played a big role in this, Rankin says, “because General Patton, who the Nazis feared as the Americans’ best general ... was in charge of what was called the First United States Army Group.” This was a force notionally of 250,000 men based in Kent in southeast England, just 20 miles across the English Channel from Calais.

“It simply didn’t exist,” Rankin says.

Another fictional army — this one British — was "stationed" in Scotland as though poised to invade German-occupied Norway.

The Allies built bogus barracks, tented camps and supply dumps for these non-existent armies. They also built rubber tanks and wooden planes and dummy landing craft to deceive the enemy’s aerial reconnaissance planes. They even replicated all the trackable services that such an army might need — in particular, simulating all the radio traffic.

“But the most important part,” says Rankin, “was that every single German spy that they thought they had in the United Kingdom were all actually captured or controlled or invented” by the British security service, MI5.

These agents were controlled by a committee of British spies in London and were sending back to Germany “an immense amount of false information.”

“One man, the great Garbo, a Spaniard called Juan Pujol, had 23 fake agents — all of which were invented — sending back this information.”

And it worked.

Hitler trusted his spies — in particular Garbo — and refused to accept that the Allied landings in Normandy were the real thing. As a result, Hitler refused for weeks to release vital reserve forces that could have proved decisive as the Allies struggled to secure their foothold in France.

“To do a proper deception,” Rankin says, “you can’t do it overnight. It has to work for years.”

“It’s like playing poker,” he adds, “you don’t let people see your cards, but if you do let them see something, you’re trying to deceive them. If you’re weak, you pretend to be strong. If you’re strong, you pretend to be weak. This is the nature of deception. You are always bluffing in some way.”

Technology has changed, Rankin notes, so it would be very difficult to pull off something like this today. But he says “probably the way that you do deceive nowadays — and this is not what you’re meant to think — is through the mass media. You can distract, if you can get stories onto television and social media.”