jam-packed with hundreds of thousands of soldiers,

all planning on a trip to the Normandy beaches in the early summer.

The job of the British Security Service, MI5,

was to make sure that, on D-Day, the Germans were looking the other way.

If they failed, tens of thousands of men would die.

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In August 1942, desperate to score a success

in the face of an unending string of military setbacks and defeats,

the Allies carried out a daring raid on the French port of Dieppe.

But the Germans were ready for them, fortified and prepared for the attack.

It was an unmitigated disaster,

leaving almost 4,000 Canadians dead on the beaches.

This catastrophe convinced everybody

that when the real invasion of France came,

the Germans must not be prepared for it, or D-Day would mean destruction.

This was, of course, going to be a little tricky.

By 90 days after D-Day, the US army alone

planned to have 1.2 million men with all of their equipment in France.

You can't just hide that under a bit of camouflage netting.

And so was born Operation Fortitude,

the most ambitious deception plan in military history.

The operation wouldn't pretend that there was to be no invasion,

that would be impossible. Instead, their goal would be

to fool the Germans about how many were coming, when they would land,

and where they would land.

Army groups were positioned around Britain to confuse the Germans.

The British 4th army was stationed in Scotland,

supposedly preparing to attack Norway.

General George Patton became the proud commander

of the 1st US Army Group

stationed on the English coast opposite the Pas de Calais,

the closest crossing to France.

The British 4th and the US 1st were somewhat unusual armies

in that there were no actual people in them.

They were completely fake, made up, non-existent.

The allies had no real soldiers to spare,

they were all needed in France for the ACTUAL invasion.

So to get the Germans to fall for this massive bluff,

the film industry was called in.

They created a brilliant illusion, an entire dummy army,

wooden aircraft, inflatable tanks, 250 fake landing craft.

Two fake corps headquarters were invented,

pouring out a constant stream of radio drivel.

At one point, King George VI even made an official inspection visit,

solemnly inspecting row after row of blow up tanks and real troops

who had bussed in for the day,

all while the daily news reel broadcast this inspection to the world.

George even managed to look thoroughly impressed.

And, because you might as well go big or go home,

this inflatable 1st US Army was presented not just as a threat, but as

the threat, the primary invasion force to be launched at the Pas de Calais.

The Germans already considered this

the most likely destination for the Allied attack;

now the Allies just had to make sure the Germans CONTINUED to believe that.

And so was born MI5's Twenty Committee,

so called for the XX Roman Numerals… Double Cross.

Double Cross fed the Germans a constant stream of highly credible intel,

a combination of utter rubbish and brilliant information

delivered just too late to be useful.

Back in 1939, the British had turned a German agent into a double agent.

He was pretty useless, but he had told MI5 everything they needed to know

about how the German secret service communicated with their agents.

The Twenty Committee used this information to create

an entire network of double agents.

By D-Day, Germany's network of spies in Britain

was owned by the Twenty Committee.

One agent was a Catalan, code named GARBO.

Garbo created a completely fictitious network of 27 agents

who bombarded the German embassy in Madrid with messages,

presenting an utterly convincing, and utterly false, picture.

The REAL attack, they said, would come in the Pas de Calais;

any other perceived invasions such as, I don't know, Normandy, whatever,

those were feints. If you hear any guns or shouting,

just relax, it's just the neighbours again. Go back to bed.

Garbo was good at his job.

So good, in fact, that the Germans awarded him with the Iron Cross

for his invaluable services.

But the danger was that even if they fooled the Germans before the invasion,

once the Allies landed, the deception would be over.

All German military strength in France would be concentrated on Normandy;

and the Allies would then no doubt be driven back into the sea.

So a series of new operations were concocted to convince the Germans

that other invasions would follow the first all along the coast of France.

Secrecy was critical.

To help the invading forces,

a complete photographic map of all of the beaches was needed.

The RAF and USAAF ran hundreds of photographic missions,

but they needed more.

So the public were asked to send in all of their holiday beach photos.

Then the toy maker Chad Valley was commissioned to take this intel

and make a vast map of the operational area

in jigsaw puzzle form for easy mobility.

The jigsaw map was delivered by two men in two pieces,

neither of them knew which was the real one,

and, once the delivery was complete,

neither was allowed to leave headquarters until D-Day was over.

The British armed forces were obsessive about security.

They arranged for 30 members of the women's Auxiliary Air Force

to dress in civilian clothes

and visit pubs close to where the British commandos were staying.

Their job: to flirt with the men and try to get them to spill

information about their mission.

To everybody's amazement and delight, the commandos kept quiet.

MI5 still worried that the news was out, though,

and that the Germans would be ready and waiting.

Back in 1942,

the crossword in the Daily Telegraph newspaper had included a clue

answered by the word "Dieppe",

just one day before the Dieppe raid had taken place.

Worrisome. And now, on May 2nd,

in the same crossword section came the clue "One of the U.S.",

and the answer was Utah,

the code name for one of the beaches the Americans were to land on.

Then on the 22nd of May, came the answer "Omaha",

another beach, and then later "Mulberry", "Overlord" and "Neptune".

All code words for various D-Day operations.

Finally, MI5 descended in a fury upon Leonard Dawe,

the school master who wrote these crosswords.

Turns out that it was all just an extraordinary series of coincidences.

But the big question remained: had all of this deception and secrecy worked?

And how would the Allies know if it had?

They needed to be able to read German communications to be sure.

The Germans used an extraordinarily complex encoding system: Enigma.

The codes were changed daily,

and there were 159 million million million possible permutations.

German messages should have been impossible to read.

But the British thought they could crack the code.

They assembled a team of brilliant mathematicians like Alan Turing

at Bletchley Park to do it.

Then the allies had a lucky break.

In May 1941, a British ship forced a German U-boat to the surface.

The Germans abandoned their sinking craft, but in the final moment,

the British sailors managed to board

and capture the most unimaginable treasure:

a completely intact code machine and codebooks.

Soon, and for much of the rest of the war,

the British were routinely reading German messages.

One estimate has suggested that the intelligence gained from this find

may have shortened the war by more than 2 years.

It was through that intelligence the Allies learned

that the Twenty Committee had been stunningly successful.

When the balloon finally went up, every key German commander

greeted the news of operations in Normandy as an invasion,

but not the invasion. Critical German focus, men and material

remained on the Pas de Calais even after D-Day,

confusing, slowing and weakening the German response.

It was little short of a miracle.

And so D-Day began for the British and Canadian forces.

The British beaches were called Gold and Sword,

and the Canadian beach, Juno.

Churchill had insisted on proper names rather than silly code names,

saying that no mother wanted to hear that her son had given his life

at the "Bunnyhug" landings.

The British parachute drop, despite massive confusion,

achieved most objectives in the face of incredible odds.

700 men and a complete glider train of artillery

were supposed to be sent to capture a critical German battery.

But when all of the equipment and most of the men were lost in the landing,

the 150 remaining soldiers,

armed only with rifles and Sten guns, took the battery none the less.

It's 7:25am at Sword beach. Mine clearing tanks thrash the sand,

sappers run beside them to disable mines and obstacles,

frequently falling to enemy fire along the way.

Then come tanks and flamethrowers

throwing themselves at the German defences on the dunes.

And only then come the landing craft to release their cargos of men.

In the chaos, the schedule collapses in only half an hour.

The beach has become a tangle of men, machinery, twisted wreckage,

a chaos of dead, dying and living.

The air is filled with the roar of gunfire.

Every moment is a drama of comedies, tragedies and moments of heroism.

But despite the cost, the landing at Sword was remarkably successful.

By 9:00pm, many units were off the beaches and moving inland.

At Juno, Canadian losses were heavier,

but they soon broke through the coastal defences, as did the British at Gold.

The landing plan had worked amazingly well, considering.

Almost everywhere, the German coast lines had been rolled up.

And the allies had complete control of the air.

The Luftwaffe was nowhere to be seen.

Now was the time to strike hard and fast into the interior,

to exploit the Germans' shock and seize the vital objectives,

including the major town of Caen. There was a period of calm

as men brewed up to recover from the initial assault,

and then they moved inland to link up with the airborne units.

Unfortunately, the beach heads behind them were a massive traffic jam.

The roads were chaos, and their armour support

was still fighting their way through that traffic to reach them.

And the Germans continued to fight hard; key strongpoints resisted

and slowed the British and Canadian infantry's advance.

At 6:00pm, a unit of the King's Shropshire Light Infantry

came under heavy fire 3 miles short of Caen and dug in for the night.

With little armoured support,

pressing the attack and capturing the town was not an option.

That night, men lay across a 60 mile stretch with their weapons,

peering into a darkness lit by flares, tracer and shells.

D-Day had been a remarkable success;

but as elite German divisions approached,

the fight for Normandy was about to begin,

and it would be harder than any could imagine.

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