(explosion) NARRATOR: It's one of the darkest moments of World War II.

Hitler invades France, smashing through Allied defenses.

CATHAL NOLAN: The French army was shattered, the British army was expelled from Europe.

NARRATOR: Leaving 400,000 soldiers hopelessly trapped on a French beach.

Mercilessly shelled, strafed...

They couldn't miss.

NARRATOR: ...and bombed.

JEFF HAWARD: You'd just lie on the sand and pray.

NARRATOR: Their only escape route blocked by a baffling new threat lurking beneath the waves.

They knew nothing about this.

They were going completely blind.

NARRATOR: But thanks to the dogged ingenuity of scientists and engineers...

It completely defeated Hitler's new secret weapon.

It was absolute genius.

NARRATOR: Through the daring exploits of pilots in their high-performance fighters.

And the bravery and perseverance of troops on the ground, 350,000 desperate men manage to escape on a makeshift armada, denying Hitler a decisive win and opening the way for America to enter the war.

HAWARD: Only the British can turn a defeat into a victory.

NARRATOR: "The Great Escape at Dunkirk."

Right now on "NOVA."

(explosion) ♪ ♪ NARRATOR: In the spring of 1940, long before America enters World War II, the German army strikes a decisive victory in France that many fear spells the end of the war in Europe almost before it has begun.

On this now-quiet beach in the French town of Dunkirk, 400,000 Allied soldiers are stranded with their backs to the sea and under merciless assault with no hope of rescue.

Newly elected prime minister Winston Churchill faces the prospect of losing the core of the British army.

The likelihood is that Britain would have had to surrender.

(explosion) NARRATOR: Senior government ministers begin to think the unthinkable.

NOLAN: It was debated, "Can we come to an accommodation with the Nazi new order?"

NARRATOR: With Churchill's leadership hanging in the balance, Hitler is on the verge of conquering France and threatening Britain.

And if Britain were to fall, it's unlikely America would have entered the war in Europe.

History might have played out very differently.

Dunkirk could have been one of the biggest military disasters in history.

But against the odds, most of the trapped men do make it back to Britain in what comes to be called the Miracle of Dunkirk.

Now a group of scientists, historians, and engineers are trying to uncover the hard truth behind this miracle.

(beeps) IAN PROCTOR: She was shaken by this massive explosion, which Hitler always referred to as his first secret weapon.

NARRATOR: They're examining newly released wartime files that shed fresh light on Dunkirk.

SIMON PARRY: We actually knew of the existence of the files, but we've never been allowed to see them.

NARRATOR: And archaeologists are digging for a lost airplane that played an essential role in the battle.

STEVE VIZARD: I don't think you've quite got all of Rolls Royce.

NARRATOR: What they uncover reveals a combination of grit, bravery, and technical brilliance that snatched total victory from Hitler's grasp.

(plane roaring) ♪ ♪ On a chilly spring day, these beaches are empty.

But there's evidence here of this coast's violent history.

A few miles north of Dunkirk, two unexploded World War II artillery shells have been uncovered.

(radio chatter) They are still lethal weapons.

(radio chatter) The bomb squad sets up a 300-yard exclusion zone.

Then they attach plastic explosives to carry out a controlled detonation.

(counting down in French) (explosion) ♪ ♪ In late May of 1940, explosions like this are a brutal reality of life for the Allied troops trapped on these beaches, as this original photograph shows.

They are surrounded by Hitler's Panzer divisions.

Above their heads, the Luftwaffe strafe and bomb them seemingly unopposed, while German artillery pound them with explosive shells.

With no sign of rescue, the men ask, "Where is the R.A.F.?"

Why are there no ships to rescue them?

How can they get out of this alive?

Things looked very different just a month earlier.

(ship horn blares) To contain the Nazi threat, Britain sends 400,000 of its best soldiers, called the British Expeditionary Force, to join with two-and-a-quarter- million French troops in northern France.

(marching) The British also deploy around 300 aircraft, including around 70 Hawker Hurricane fighters.

Hitler had already occupied Austria and Czechoslovakia and then invaded Poland.

NOLAN: The invasion of Poland, I would say, was confirmation that Hitler could not be stopped short of force.

There was no dealing with Hitler; you had to fight.

NARRATOR: The Allies' aim is simple-- stop Hitler in his tracks, prevent a Nazi invasion of France at any cost.

The Allies are convinced Hitler will invade through central or western Belgium.

And that is where they mass to stop him.

They believe the terrain at the eastern end of the border is impassible to tanks.

It's a good plan.

It actually was a very good plan.

It just didn't work.

(propeller chugging) NARRATOR: On the morning of May 10, 1940, Hitler makes his move.

(airplanes droning) He does exactly what the Allies expect and invades Belgium.

Allied forces move north to stop him.

On the evening of the same day, Winston Churchill becomes prime minister of Great Britain.

The good news for him is that so far, the Allied plan is working.

(tank rumbling) But Hitler's Belgian attack is a feint.

Three days later, a bigger force of German Panzer tanks breaks through the French border to the southeast, where the Allies had thought it impassable.

This two-pronged German attack now moves with a speed and violence the Allies are totally unprepared for.

The Allies were still moving forward into Belgium when the Germans were coming behind them, and the Allies are going to find themselves trapped.

NARRATOR: The Allies crumble under the ferocity of the Nazi attack and, along with thousands of civilians, begin a chaotic retreat.

In just eleven days, the Allies have been completely encircled, in a rapidly shrinking territory, with their backs to the English Channel.

The new prime minister faces a catastrophe.

To save the men, foreign secretary Lord Halifax, urges Churchill to open peace negotiations with Hitler.

Churchill is dismissive.

The trapped men have no option but to fight on.

In France, surrounded by the Nazis, thousands of British and French troops begin pouring into Dunkirk.

Their situation is desperate.

The whole of Dunkirk was on fire.

GARTH WRIGHT: There's this great ball of smoke.

going up from the oil tanks.

HAWARD: I remember the wall falling down.

NARRATOR: Jeff Haward was a 20-year-old gunner.

As he reaches the Dunkirk beaches, he runs into two officers.

They said "Right, carry on down to the beach, and someone will be waiting there to tell you what to do."

But of course there was no one waiting there, were there?

NARRATOR: The Allied collapse has been so rapid that British military officials are still scrambling to put a rescue plan in place and find ships to get them home.

By May 23, parts of the German front line are less than 20 miles from Dunkirk.

(artillery firing) Death or capture now seems inevitable.

Then something extraordinary happens.

The German advance suddenly stops.

NOLAN: Once the Allied forces were inside the Dunkirk perimeter, from the German point of view, they were a defeated force.

And the Germans needed to halt, because they were outrunning their supplies.

They were outrunning their infantry.

NARRATOR: Hitler's deputy, Herman Goering, is the commander in chief of the Luftwaffe.

He convinces Hitler that they can finish off the Allies without the overstretched German army.

NOLAN: Hermann Goering was an extraordinarily vainglorious man.

He persuaded Hitler that the Luftwaffe could move in and solve the problem all in one blow.

NARRATOR: The British and French troops have managed to hang on to some artillery during the retreat.

Taking full advantage of the halt, they set up defensive positions in a 30-mile perimeter around Dunkirk.

That might hold back the German army for a while, but Goering's Luftwaffe easily flies right over these defenses.

The men trapped on the beaches are easy targets.

(machine guns firing) They machine-gunned us a lot.

NARRATOR: The aircraft they fear the most is the Stuka dive bomber.

Stukas attack in a steep dive, literally flying straight at the target and releasing the bombs at the last minute.

(explosion) WRIGHT: They aimed their plane at their target and they couldn't miss.

NARRATOR: Stukas have uniquely shaped gull wings that give the pilot a clearer view of his target.

(plane siren blaring) And fitted to the fixed undercarriage of the Stuka is a siren that sounds as it starts its attack dive.

The Germans call it the Jericho Trumpet.

(plane siren blaring) They used to make a terrible screeching noise coming out.

It was psychological, I think, to try to frighten you, which it did.

(siren blaring) That bastard.

Noisy sod.

I hated them.

HAWARD: You'd just lie on the sand and pray.

I don't think there were many atheists at Dunkirk.

♪ ♪ NARRATOR: One of the secrets of the Luftwaffe's ability to inflict so much damage is that they are able to protect slow bombers, like the Stukas, with a superior fighter aircraft, the Messerschmitt 109.

The 109 is a single-seater, all-metal design.

It is powered by an inverted V12 Daimler Benz engine, producing more than a thousand horsepower.

At Dunkirk it is armed with a 20-millimeter cannon in each wing and 7.92-millimeter machine guns in front of the pilot.

It is faster in level flight, in turns, and in climb than most of the British Royal Air Force fighters.

♪ ♪ The best front-line R.A.F.

fighter in France is the single-seater Hawker Hurricane.

At the start of the war, it is the workhorse of the R.A.F.

(engines roaring) The airframe is made of steel tube, aluminum, and wood and covered in a fabric skin.

It is the pinnacle of design practices dating back to the First World War.

It is armed with eight Browning .303 machine guns.

The Hurricane's construction is outdated, but it is a stable gun platform that can inflict great damage on bombers like the Stuka.

And with a good pilot, a Hurricane can challenge a German 109 in a fight.

But German pilots have more experience than their R.A.F.

counterparts.

They have already flown combat missions in Spain during the civil war there.

(explosions) In the early days of the Battle of France, R.A.F.

Hurricane and bomber squadrons sustain huge losses from a combination of less-experienced pilots, less-advanced technology, and overwhelming German numerical superiority.

As the men on these beaches are hit by wave after wave of Luftwaffe attacks, there is little sign of the R.A.F.

To these men it looks like the air war is already lost.

WRIGHT: They were kicking up hell about "Where's our planes?"

when we were under such terrible stress with the endless attacks we were getting from the air.

"Where the hell's our air force?"

NARRATOR: A very different story about the R.A.F.

can be found at the National Archives in London, where World War II files have recently been released.

Historian Joshua Levine has come here with aviation expert Simon Parry to investigate.

We actually knew of the existence of the files for many years, but we've never been allowed to see them.

NARRATOR: The newly released documents are R.A.F.

casualty files, one of which was started every time an airman failed to return from a mission.

Approximately 200 of them cover the period of the Dunkirk operation.

Here's the file for one pilot, Sergeant Jenkins.

And you can see he's recorded missing 29 May 1940.

Blue Leader reports seeing Blue Two, Sergeant Jenkins, on fire and diving down.

Sergeant Jenkins bailed out at 5,000 feet and made a landing in the sea eight miles north of Dunkirk.

NARRATOR: File after file tells the same story.

The R.A.F.

is fighting around Dunkirk, taking on the Luftwaffe, despite sustaining heavy losses.

The files contain very personal evidence.

In one, a letter from a mother in America asking for news of her son, a fighter pilot reported missing after attacking enemy bombers heading for Dunkirk.

San Diego, California, the officer's mother.

"To date, I have received no news or information concerning my son, except that he is missing."

So the other side of the Atlantic equally, they are awaiting information.

JOSHUA LEVINE: Do we know what happened?

They've written: "It is regretted that no further news "has been received of your son, "Pilot Officer Richard Dennis Aubert, since he was reported missing on the 24th of May 1940."

LEVINE: So it's just a blank.

Yeah.

He just ceased to exist.

NARRATOR: Using the detail revealed in the files about crash sites, it's now possible to piece together the true story of the R.A.F.

's role at Dunkirk and explain why the troops think the R.A.F.

has deserted them.

Each R.A.F.

symbol represents a plane that has been shot down.

It shows that the heaviest losses are inland, where soldiers on the beach wouldn't be able to see them, as pilots fight to protect the retreating troops from the approaching German air force.

Their job, their task, if you like, was to give the troops on the beach a maximum chance of getting away and to hinder the Luftwaffe.

And so the RAF succeeded in their aim.

NARRATOR: The fact that the R.A.F.

is often fighting out of sight inland of Dunkirk isn't the only reason the troops don't see them.

PARRY: The battles were being fought at great height, four or five miles above them, and there's no way that the troops on the beach could see what the R.A.F.

were doing.

NOLAN: The R.A.F.

did not withdraw from the battle over Dunkirk.

Those R.A.F.

fighters who continued to engage the Luftwaffe did so under increasingly hazardous and outnumbered conditions.

And it was nothing but heroism in the skies.

NARRATOR: Although the R.A.F.

is often outnumbered and outclassed, over Dunkirk, the Luftwaffe is now sustaining losses too.

(machine guns firing) (plane diving) And for the first time in large numbers, a different British plane is putting pressure on the Germans.

(fire crackling) (soldiers talking) ♪ ♪ Aviation historian Simon Parry believes this field in the east of England could be the crash site of one of the R.A.F.

's finest fighters, a Mk.

I Supermarine Spitfire.

PARRY: What we've been able to gather so far from the archives is this particular plane flew over the beaches of Dunkirk, and it was actually involved in combat.

NARRATOR: If they were to recover a Spitfire that flew at Dunkirk, it would be an incredibly rare find.

(metal detector buzzing) The experts begin by surveying the area for magnetic anomalies.

Steve Vizard has been restoring Spitfires for 33 years.

He knows every nut, bolt, and rivet.

VIZARD: Anything that's causing the readings is going to only be ferrous.

So it could be a piece of armor plate, undercarriage leg, anything steel.

But we could be picking up an oil drum, or general rubbish off the farm.

That's always the risk.

NARRATOR: When it crashed, on July 4, 1940, the aircraft they are searching for was being flown by a rookie pilot on a routine patrol along the English coast.

(engines humming) But if it's the plane they think it is, just a month before, under command of another pilot, it flew at Dunkirk.

But is this the right plane?

It's not until you start digging you actually find out what... what actually happened.

NARRATOR: If they're in the right place, the loose earth used to fill in the crater left by the crash should show up clearly against the surrounding clay.

VIZARD: Well, you've got the line coming through now.

As you can see, which is coming round the edge here, the more clay colored.

And you can see the darker disturbance in the middle.

♪ ♪ Dan, you want to grab it?

That's good.

It's a metal propeller blade.

One of the three de Havilland blades that it had.

Three metal blades on the Mk.

I Spitfire, and that's one part of one of them.

NARRATOR: It confirms that this is the right crash site.

And more evidence appears as they dig deeper.

That's a little bit of the V-shaped section, top longeron, that goes down the spine of the fuselage behind the pilot's head, from the canopy back to the tail.

That's just a sort of microscopic part of the radically different way that the Spitfire was built compared to any other airplanes of that time.

NARRATOR: Unlike older aircraft, the frame of a Spitfire was too lightweight to support the plane in flight.

An additional key part of its strength came from the aluminum skin that was riveted to the frame.

This made the finished aircraft lighter and stiffer than the old wood and canvas construction of previous British aircraft.

Engineers call it a semi-monocoque.

(engine droning) The cutting-edge construction allowed engineers to form the skin into complex aerodynamic shapes to achieve greater speed and maneuverability.

VIZARD: We always say, working on them, there's not a straight line on a Spitfire-- everything is curved or double curved, which up to then had never been done before.

NARRATOR: The wing supports, known as spars, were also far thinner than in any previous fighter, and this allowed Supermarine to design much slimmer wings with a distinctive elliptical shape.

VIZARD: Where the Spitfire really did outperform pretty much anything of that period was in its maneuverability.

The design of the elliptical wing gave it a much... much, much better stalling characteristics, and it could turn inside virtually any other airplane.

NARRATOR: In combat, the Spitfire could turn 25% faster than a German 109.

(engine droning) After six hours hard work, they reach the heart of the plane.

VIZARD: I think that's an exhaust stub, which is obviously the side of the engine.

(scraping) Can you see the "Rolls"-- R, O, double L, S?

You haven't quite got all of "Rolls Royce."

GEOFF: It's actually not in bad nick.

♪ ♪ Rolls Royce Limited England, Merlin number three.

NARRATOR: The Spitfire's supercharged V12 Rolls Royce Merlin engine produced more than a thousand horsepower.

This engine, combined with the Spitfire's advanced airframe, gave it a climb rate that matched or exceeded the 109 and an equivalent top speed.

VIZARD: The Spitfire did everything that you wanted it to do, almost as you thought it.

And as is often said, you don't actually get into a Spitfire, you strap one on.

NARRATOR: But how did this plane, that survived the battle at Dunkirk, end up crashing?

From the evidence of the wreckage and the position of the engine, they now believe that the new pilot was fighting to save his Spitfire until the very end.

Had the plane crashed vertically, the engine would have been buried nose down in the ground.

As it was, the engine was flat, so the aircraft had gone in at that angle.

It proves that.

NARRATOR: Their best guess is that the pilot became disoriented in the clouds and came out of them relatively close to the ground.

Although he tried, there wasn't quite enough time for him to react.

You have to say if the poor guy had probably been ten foot higher, he might have got away with it.

Britain had declined to send a single Spitfire to France.

Now Churchill commits at least fifteen Spitfire squadrons to defend the troops.

For the first time, the Luftwaffe faces Spitfires en masse.

But will it be enough to allow the soldiers to escape?

R.A.F.

pilots certainly believe they are making a difference.

During Dunkirk, squadron leader Geoffrey Stephenson reports that his squadron of just 12 Spitfires holds off an attack force of 50 German aircraft.

(engine droning) Stephenson is in action near Dunkirk on May 26, 1940, when his Spitfire, N3200, is shot down in combat.

Stephenson survives the crash, and for a time in the summer of 1940, his Spitfire is a tourist attraction for the occupying German soldiers.

But France does not become its final resting place.

In 1986 the wreckage of Stephenson's aircraft is recovered and returned to the U.K. 30 years after it was pulled from the sand, this is the actual Spitfire Stephenson flew over Dunkirk, carefully restored in every detail to her original 1940s specification.

JOHN ROMAIN: So here she is, Steve, N3200.

VIZARD: Yeah, Geoffrey Stephenson, fresh from Dunkirk.

Fresh from Dunkirk.

NARRATOR: John Romain has spent 28 years flying Second World War fighter planes.

VIZARD: She's exactly the same markings now as... Yeah, as she was then.

...as she was.

Exactly, down to the last detail.

And of course this airplane was lost with one bullet going through one of these pipes.

And, um, that was enough to...

Causing it to overheat and... Overheat, and he then knew he wasn't going to get back to England, so that's why he bellied the airplane down on the beach.

NARRATOR: As Spitfire pilots learn to get the best out of their new aircraft, they quickly add personal modifications to keep themselves alive.

ROMAIN: The early airplanes, they didn't have rear-view mirrors in them.

So once they started to get into combat, they realized that they needed a mirror.

And so before they could start putting them on the airplanes in production, all the pilots started running around producing their own mirrors, and this was Geoffrey's mirror from his MG. NARRATOR: Kesselring, the general in charge of the German air force at Dunkirk, admits that the Spitfires were making Luftwaffe air operations difficult and costly.

But it still isn't an even fight.

The British have to hold back most of their fighters for the expected invasion of the U.K. NOLAN: Had the decision been made to deploy everything the R.A.F.

had, they could have gone in there and provided much more extensive air cover.

Would that have saved some lives?

Probably.

But how many planes would have been lost?

And that calculus had to be made that these planes were critical to the defense of Britain itself.

The logic I think is impeccable, and the decision was correct.

(alarm ringing) NARRATOR: Resources are so limited that fewer than 200 R.A.F.

pilots have to fly nearly 2,800 missions across just nine days.

Over the course of the Dunkirk defense, they shoot down 78 German aircraft, but at the cost of a roughly equal number of their own planes and pilots.

The sacrifice of the R.A.F.

pilots alone cannot save the soldiers trapped at Dunkirk in a rapidly deteriorating situation.

I can remember for the first time smelling death.

NARRATOR: In May 1940, Garth Wright was a 20-year-old gunner in the Royal Artillery.

It was this smell of rotting corpses.

NARRATOR: Don Hall was just 19 years old.

The smell, oh, terrible.

What with bodies have been laying there for some considerable time.

NARRATOR: Getting rescue ships to the trapped men will be tough.

For nine months, Hitler has been laying secret undetectable sea mines that have already sunk hundreds of thousands of tons of Allied shipping.

By the time of Dunkirk, over a thousand of these deadly weapons had been laid in the waters around Britain.

Today the light cruiser HMS Belfast is preserved as a floating museum in London.

She entered service in August 1939.

Three months later, she encounters the new Nazi secret weapon.

Dunkirk expert Joshua Levine has come here to meet curator Ian Proctor.

PROCTOR: HMS Belfast on the 21st of November 1939 was leaving the Firth of Forth.

At 10:58 in the morning, she was shaken by this massive explosion.

The ship lifted out of the water, and then as it settled down there was a really big shuddering.

And when she lifted out of the water, she broke her back, which basically means that the keel was distorted.

The power went out, the engine room, which we're in now, started filling with water, and the captain in his report assumed, that there had... well, naturally, that they had been struck by a torpedo fired from a submarine, but as it turned out, that wasn't the case.

What was going on, what had happened?

The ship had actually accidentally detonated a mine, primarily... or specifically a mine which Hitler would refer to as his first secret weapon.

NARRATOR: Belfast suffers no blast damage.

Instead she's been hit by a powerful shockwave.

This gives scientists a vital clue.

(explosion) The mines can sense a ship is close without touching it.

Churchill orders the recovery of one of these deadly weapons at any cost.

And the day after the Belfast is hit, the British have an astonishing stroke of luck.

A German aircraft accidentally drops one of the secret mines on a Thames mudbank.

Scientists from the Naval Mine School, HMS Vernon, are scrambled to defuse it.

This is D.E.M.S., the British military's center for explosive mines.

Dr. Simon Foster, a physicist, is here to discover the secrets of the Nazi supermine, using a rare example of an original German mine.

Petty Officer Nigel Froude has 27 years of expertise in the technology of naval mines.

FOSTER: Did they have any idea of what's inside?

Absolutely not, so they knew nothing about this, so they were going completely blind.

NARRATOR: Their first task was to defuse the mine by removing the detonators designed to trigger its 660 pounds of explosives.

FROUDE: So they removed this plate.

It was obviously more than hand tight.

So it literally was a big screwdriver, hammer, just slowly tapping it, to loosen it up, until they were physically able to undo it with their hands.

So, they-they had decided, with a live mine, the best way to get inside it was to hammer away with a screwdriver?

It was gentle taps, but, yes, effectively, it was a hammer and a screwdriver.

And had they any kind of inkling as to what this mine was doing?

Had they seen any other mines that they had a kind of understanding of what they might find?

These guys were trained in bomb disposal, but this was completely new to them.

So a lot of it was just trial and error.

The bravery of these guys is just phenomenal.

NARRATOR: The defused mine was taken by truck to the Naval Mine School and disassembled.

This is the key to it all.

Inside here, this mine is going to reveal its secrets.

Yeah.

The grand reveal.

This would have been the moment of truth.

And there we have it.

This is the trigger that's going to make the whole mine go bang.

If we now remove this plate here, we can see the trigger just inside there.

If I just move it with this screwdriver, you can see the movement of the switch there.

And that's just like a seesaw.

Exactly, just like a seesaw.

It moves, makes the circuit, and the mine would go bang.

Now, what's making that seesaw move?

Magnetism, that was the big secret.

NARRATOR: Scientists and engineers investigating the mine knew that understanding exactly how the magnetic trigger works was the first step in neutralizing Hitler's deadly weapon.

FOSTER: The seesaw inside the German magnetic mine, is a dip compass, and we've got our own version here.

Now, unlike a normal compass that moves left or right to indicate the magnetic field, this actually moves up and down.

NARRATOR: A dip compass measures how much the earth's natural magnetic field points downwards into the ground.

This field is generated by the earth's molten metal core.

In Europe, the earth's north magnetic field points down, into the earth, at an angle towards the core.

Simon uses a steel plate to represent the hull of the ship.

FOSTER: The earth's magnetic field that's all around us, it finds it easier to pass through this steel plate than it does in the surrounding area.

And this plate here is actually concentrating the magnetic field lines.

The magnetic field passing through this is going to be more concentrated here than it is out here.

This, this is almost like a lens for magnetism.

And as you can see, as it passes over the compass, (beeping) it's going to trigger the mine.

NARRATOR: This concentrating effect turns a steel ship into a gigantic magnet with its north pole under the ship.

It is this strong north pole which triggers the mine.

If they create an artificial magnetic field that generates a south pole under the ship instead, the mine will not go off.

The first method that they came up with was called degaussing.

If you have a line of cabling like this and run some current through it, it's going to create a small magnetic field in the opposite direction to the earth's magnetic field.

Now, if I take my bit of steel and place it over the mine as we did previously, hopefully the mine will no longer be triggered.

And this is how they solved the problem.

They wrapped a long line of cabling around the outside of the ships, and run huge currents through them, creating a magnetic field that actually countered the concentrating, the lensing effect of the ship's hull and prevented the magnetic mine being set off.

♪ ♪ NARRATOR: But there is neither the cable, nor the time, to fit magnetic equipment to enough ships to evacuate 400,000 men from the Dunkirk beaches.

But there is a solution.

It comes from a Canadian scientist at HMS Vernon, Charles Goodeve.

He is a brilliant inventor and comes up with a simpler plan to magnetize an entire ship.

He calls it wiping.

FOSTER: If it take this coil and pass more power through it, I can create such a big magnetic field that I can actually magnetize this piece of steel.

And this was done to ships in the fleet.

They dragged huge cables around the outside of the ships and actually magnetized the hull.

Now, when I pass this over our triggering mechanism, it won't trigger off the mine.

And just to prove that I've actually wiped this, I'm going to use our original piece of steel as an unwiped ship and pass that over, (beeping) and you can see it's still setting off the mine.

This actually prevented any of the mines going off.

Degaussing and wiping completely defeated Hitler's new secret weapon-- it was absolute genius.

♪ ♪ NARRATOR: This rare wartime color footage shows a ship actually being wiped using Goodeve's technique, with a huge cable carrying thousands of amps of electricity.

The protection only lasted six months, but it was quick and it was easy.

To save the men at Dunkirk, 400 ships are wiped over just four days.

With enough ships protected from mines, there is now a fighting chance of getting them out alive.

♪ ♪ On May 26 at 18:57, Churchill rolls the dice.

He orders a full-scale evacuation at Dunkirk.

It is called Operation Dynamo.

Churchill is gambling vital Navy ships in order to save the trapped soldiers.

It is the biggest maritime evacuation in history.

It doesn't come a moment too soon.

On the same day, after a two-day pause, Hitler rescinds the German army's halt order and the tanks begin to advance towards Dunkirk again.

As the Germans realize that the evacuation is underway, they begin to pressure the perimeter increasingly.

So of course, the defense of the perimeter becomes critical to the success of the evacuation.

♪ ♪ NARRATOR: From the defensive ring the Allies have built around Dunkirk during the halt order, using what is left of their equipment, French and British troops put up fierce resistance in a rapidly deteriorating situation.

(artillery fire) (explosion) On May 27, 1940, with little good news from Operation Dynamo, Lord Halifax once again tells Churchill that to save the troops, they must begin peace negotiations with Hitler.

Again, Churchill resists.

Halifax threatens to resign, a move that could bring down Churchill.

Britain now faces losing its greatest wartime leader, and, on the beaches of Dunkirk, most of its army, as well.

Operation Dynamo does not start well.

The Luftwaffe has destroyed Dunkirk harbor.

Only the harbor's mile-long breakwater, known as the Mole, is still standing.

It is barely two yards wide, and its wooden structure back then was not designed for mooring large ships.

Evacuating hundreds of thousands of men this way will be tough, but it's all they have.

Of all the snap decisions that were, that were made on the spot, for me, that is the greatest decision, the idea to, to call this great big breakwater into service as something it was never meant to be.

NARRATOR: On May 27, ships begin loading men from the Mole.

By the next day, more than 18,000 men have been taken off.

The Luftwaffe realizes what the British are up to almost immediately.

Heinkel bombers and the Stukas target the Dunkirk Mole and the ships moored alongside it.

WRIGHT: It was bombed and shattered in places, but the bridge with makeshift planks and things like that.

HALL: Half of it had been blown away, so you went in single file down that, straight onto the boat.

NARRATOR: Despite the repeated attacks, two-thirds of those who escape Dunkirk do so via the Mole.

Not everyone is lucky enough to scramble onto the overcrowded breakwater.

Instead, those men must evacuate from the beaches.

But the Dunkirk shoreline is too shallow for large ships to get close in to shore.

PAUL REED: When we see the iconic images of men on beaches like this, we do see, well, almost orderly queues of men going out to the ships, going out to the point where they can swim out to a vessel and get away.

NARRATOR: Getting troops out this way is slow and dangerous.

Loading just 600 men is taking up to eight hours.

The desperate men in the water are an easy target.

These queues going out to the small boats, they were just bombing them all along the beaches.

Their job was to get rid of as many as they could, I suppose.

NARRATOR: The Allies have been forced to abandon their heavy equipment-- just getting the men out is tough enough.

Army engineers realize they can use some of the abandoned trucks, known as lorries, to save the men.

REED: They came up with the idea of a lorry pier, where they drive lorries onto the beach, and we've got a captured German photo here showing such a lorry pier.

They drive the lorries onto the beach, line them up side by side, front of the lorry here, rear of the lorry here, and then put planking on the top, so the planking enabled them to walk across the roofs of the lorries, get down there quickly.

At high tide, the water would be lapping up both sides, and there'd be lots of little boats available to then ferry them from the far end of the lorry pier out to sea to the bigger boats.

NARRATOR: It all helps, but by May 28, the Germans are closing in on Dunkirk, and only 25,000 of the trapped men have made it to Britain.

As the crisis grows, the conflict between Churchill and Halifax reaches its climax.

Churchill goes for broke, with an impassioned speech to his cabinet to fight on regardless.

His gamble pays off-- the cabinet falls into line.

Halifax is neutralized.

He never demands negotiations with Hitler again.

♪ ♪ But if the evacuation fails, the finger of blame will point squarely at Churchill.

To prevent that, every available British ship is now racing across the channel carrying troops away from Dunkirk.

WRIGHT: The feeling is indescribable.

I thought, "Dammit, I'm going to make it."

NARRATOR: But just getting off the beaches doesn't guarantee safety.

Even if you get on a large warship that seems stable and solid compared to where you've been standing on the beach for several days, that warship is under threat.

That warship might hit a mine.

That warship can be bombed.

♪ ♪ (anti-aircraft gunfire booming) (explosion) The Luftwaffe is just as content to kill you on a ship as they are to kill you on the beach.

♪ ♪ NARRATOR: Just before midnight on the 28th, the destroyer HMS Wakeful leaves with around 640 rescued soldiers aboard.

They have gotten off the deadly beaches.

But they never reach the safety of home.

♪ ♪ This is HMS Wakeful today.

She lies 80 feet below the surface of the English Channel.

77 years of marine growth hides much of the destroyer, but it is still possible to make out unopened crates of ammunition for the ship's guns and even an old gas mask.

Inside the wreck are the bodies of more than 600 men.

Around 1:00 a.m., already badly damaged by air attacks, Wakeful is hit by a torpedo.

Wakeful was basically broken in half.

She, she broke into two pieces.

And what you were left with, had you been there, what you would have seen, were two ends of the ship poking out of the water.

The upshot of that was that when the ship went down, these people basically didn't have a chance, and they drowned almost immediately.

There was one soldier who had been on deck having a cigarette.

He got away-- all the rest were drowned.

NARRATOR: The captain of the Wakeful had sent all the evacuees below decks to keep the ship's weight low down and make her more stable in fast avoidance maneuvers.

This is standard practice for ships trying to evade attack.

Because Wakeful sinks in less than a minute, the 600 soldiers locked below decks have no chance of escape.

This is part of the human cost of Operation Dynamo.

♪ ♪ (explosion) Across the whole evacuation, the Allies lose more than 230 ships.

Some are sunk in the channel, but many never make it off the beaches.

Even today, after winter storms, when the tide is unusually low, the remains of some can still be seen.

This is the Crested Eagle, a paddle steamer built to carry sightseers down the River Thames.

It was destroyed by Stuka dive bombers.

Around 300 men were killed in the attack.

♪ ♪ Despite the continued loss of shipping, Churchill has rolled the dice, and there is no option but to press on.

Even with the improvised lorry piers, ships are still struggling to get troops off the shallow beaches.

To save as many as possible, the Royal Navy sends in a flotilla of small pleasure crafts that can get closer in to shore.

Yachts, fishing boats, launches, even a Thames fireboat, risk all to save the trapped men.

They shuttle them from the shallow beaches out to larger vessels before returning to the U.K. with as many survivors as they can carry.

They become known as the "Dunkirk Little Ships."

HAWARD: Those little boats came, and when you read of what they did, it was marvelous.

NARRATOR: More than 100 of the original Little Ships used at Dunkirk still survive today.

Although the Little Ships saved relatively few soldiers themselves, they become a powerful symbol of Churchill's determination to beat the odds, using anything and everything he can lay hands on.

It's also a sign of just how desperate the situation is.

♪ ♪ For nine days, sailors, airmen, and civilians risk life and limb to save the trapped men.

And Churchill's gamble pays off beyond all expectations.

Operation Dynamo saves 200,000 British troops and nearly 140,000 French soldiers who were trapped at Dunkirk.

HAWARD: I could feel the boat going up and down with the waves.

I was so tired, I fell asleep.

Next thing I knew, someone was shaking me, saying, "Wake up, we're coming into Folkestone."

NARRATOR: The survivors receive a hero's welcome.

But the British lose 68,000 soldiers in France, 17,000 in the evacuation alone.

40,000 troops, mostly French, are taken prisoner at Dunkirk.

NOLAN: As an historian, I have to tell you it was a crushing military defeat.

The French army was shattered.

The British army was expelled from Europe.

If you are German, and you look at that, you don't say, "Oh, my God, we didn't capture 300,000 men."

You say, "We smashed Britain, we smashed France.

We've won the war."

Which is what many of them did think.

NARRATOR: But given how much worse it could have been, the British consider it a triumph of sorts.

HAWARD: Only the British can turn a defeat into a victory.

NARRATOR: Winston Churchill has got his win.

Instead of reporting the greatest military disaster in Britain's history, he tells the British people that it is "a miracle of deliverance."

It is not by any measure a military victory, but it is a victory over those that wanted to give in to Hitler.

♪ ♪ NOLAN: Churchill's greatness is that he persuaded the cabinet and the country not to quit the war, that even though we cannot see a path to victory, we must stay in this fight until, frankly-- and he was right about this-- other great powers came to their senses and saw the Nazi threat for what it was.

Therefore, fight on until the Americans take their head out of the sand and realize that they must come and rejoin the fight, that their security lies along the Rhine just as the British does.

(engine starting) NARRATOR: In 1940, Geoffrey Stephenson's Spitfire, number N3200, was shot down while protecting British soldiers.

Now, for the first time in nearly eight decades... (plane engine roaring) ...it has returned to the scene of its most famous battles: Dunkirk.

In his speech after Dunkirk, Churchill acknowledges the human cost of the evacuation.

He praises the success of the R.A.F.

's Spitfires and Hurricanes against the Luftwaffe.

And he talks about the dangers of the magnetic mines that science and technology had overcome.

Less than a month after becoming prime minister, the courage of sailors, soldiers, and airmen, and the dedication of scientists and engineers, hand Churchill the propaganda victory that he so desperately needs to maintain the fight against the Nazis.

That is the true miracle of Dunkirk.

We can say objectively and analytically that the phrase "Miracle of Dunkirk" is a propaganda phrase, because it is.

It's not a bad propaganda phrase.

And if you're intending to fight on against Hitler, and this helps you rally the nation, it is a pretty darn good propaganda phrase, actually.

WRIGHT: We were beaten and we came back.

We lived to fight another day.

It was a miracle at Dunkirk, all right.

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