Original subtitles

- [Narrator] A message intercepted from the Germans.

- The British are in possession

of an essential piece of intelligence.

- [Narrator] That could save Britain in its darkest hour.

- Germany want Mexico to go to war

against the United States of America.

- [Narrator] For World War I's greatest code breakers,

deciphering the message is just the beginning.

- The British have got this information

by spying on the Americans.

- [Narrator] Britain's very survival

rests with a team of eccentric geniuses.

- He ran down the corridor and said,

"Do you want to bring America into the war, sir?"

- This was one telegram

that changed the entire course of modern history.

[dramatic music] [explosions rumble]

- [Narrator] These are the codes that changed our world,

bizarre markings, random letters, and numbers,

words that make no sense,

[dramatic music]

but cracking them unlocks military secrets,

decodes ancient civilizations,

and reveals enemies in our midst.

[dramatic music]

Now we uncover how they were decoded,

the genius minds that broke them,

and the secrets they reveal.

[dramatic music]

[explosions rumble] [gunfire blasting]

January 16th, 1917.

Britain and Germany have been locked

in a seemingly unending war for two and a half years.

The fiercest fighting has played out

on a series of battlefields

spread across hundreds of miles

of Northern France and on into Belgium.

[explosion rumbles]

Now, with almost two million soldiers dead on both sides

and morale at an all time low,

Britain's chances of turning the war around

are looking hopeless.

- The bloody cost of the war

has become apparent to everyone.

The casualties so far have been unprecedented.

Clearly this pace of slaughter

is not sustainable for either side.

One or the other is going to have to find a new strategy,

otherwise, this is just gonna be a slaughter without end.

[dramatic music] [explosion rumbles]

- [Narrator] Then,

at Britain's lowest point in the Great War,

a remote telegram station

on the westernmost tip of Britain

intercepts a suspicious coded message.

[telegraph beeping]

Unknown to the telegram operator,

the message conceals Germany's latest strategy.

Devised by the fiercely hard line

German foreign secretary Arthur Zimmermann.

- If you look at these codes,

you would just see a string of numbers,

which would be quite meaningless

and apparently quite random as well.

- [Narrator] The Allies have intercepted a communication

from a top German government official.

It's in code and not just any code,

the most secret code that Germans possessed.

- [Narrator] Convinced his code is un-crackable,

Zimmermann has sent the telegram via open diplomatic cables.

[telegraph beeping]

But he's underestimated Britain's intelligence capabilities.

[dramatic music]

Located deep within the old Admiralty building

in Whitehall, London,

is a top secret organization known as Room 40.

- This was a pioneering intelligence operation

looking at mainly Naval signals,

tactical information to see where the shipping roots were

and where the Germans were forming.

But increasingly they began to pick up other signals,

looking at different aspects

of the German war effort and strategy.

- And they're particularly interested

in these secret messages sent by the Germans.

Code making and code breaking

have arrived at modern warfare, and they're going to stay.

[suspenseful music]

- [Narrator] Room 40 is made up of a brilliant

if eccentric mix of linguists,

classical scholars, and puzzle addicts,

and they're about to prove vital in the war against Germany.

- This is their real strength

that it's a diversity of expertise,

a diversity of brain power.

Different people, solving puzzles in different ways

that can lead some really innovative solutions.

- You're dealing with a small team of people,

half a dozen at most, in one office,

who would be quietly working with pencil and paper,

trying to understand these messages

and working out the way in which decode worked.

- [Narrator] Members of this crack team

include Nigel de Grey, a brilliant cryptographer,

so mild-mannered, they call him the dormouse.

Alfred Dilwyn Knox, or Dilly,

a classic scholar whose best work comes while soaking

in his specially installed bathtub in Whitehall.

And the man heading up this groundbreaking team

is no less eccentric.

The director of British Naval intelligence,

Captain Sir Reginald "Blinker" Hall.

- Blinker Hall,

who was known to those who worked with him as Blinker

because he had a persistent facial twitch

was not a professional cryptographer.

He was originally a captain of a battle cruiser.

He was a Naval man, but because of ill health,

they had to assign him to other duties,

and I'm sure he saw this as a step down

going from being master and commander

of one of the great warships at sea

to riding herd over a bunch of puzzle experts

and mathematicians.

But it turns out that what Blinker Hall

is going to achieve with Room 40

is worth a fleet of battle cruisers.

[telegraph beeping]

- [Narrator] Blinker Hall's Naval background

means he knows only too well the importance of codebreaking.

Intercepting and decrypting enemy codes

can determine how battles are fought and won.

- He could see that signal's intelligence

was particularly important

in providing a detailed view of the enemy,

but also provide information

which could be used to wrong foot the enemy as well.

- Forewarned is forearmed.

- [Narrator] By the time

the mysterious German telegram lands on his desk,

Blinker Hall's crack codebreaking team

has already more than proved its worth to the war effort.

[dramatic music]

[suspenseful music]

Two years earlier,

they managed to decode an encrypted message.

Revealing the German high seas fleet was planning an attack

on British fishing vessels in the North Sea.

- There was a plan afoot for the German Navy

to set sail and to attack British trawlers,

which were thought to be passing on information

part of the intelligence network.

- [Narrator] The early warning courtesy of Room 40

meant the British Royal Navy

was able to intercept the German ships

long before they reached the trawlers.

- This led to the Battle of Dogger Bank,

which was a victory for the British.

[explosions rumble]

- [Narrator] Two German ships were sunk

and nearly a thousand German sailors lost.

This early success cemented Room 40's position

as a key weapon in the war against Germany.

[dramatic music]

[telegraph beeping]

But codebreaking is still a very new military weapon.

[explosion rumbles]

At the very start of World War I, it was a different story.

A tragic mistake just months after war broke out,

highlighted the need for secret coded transmissions.

[dramatic music]

In September, 1914,

the German Army was advancing on Paris

intent on capturing the city,

but German wireless operators made a fatal decision

that cost hundreds of lives.

- The Germans made incredible inroads.

They're on the outskirts of Paris, but there's a problem.

They're advancing so quickly

that they don't have time to encrypt the messages

that they're sending back and forth,

and they end up sending a lot of vital information

right over these radio waves.

- [Narrator] As the German military plans

were openly broadcast,

operators at an interception station

located high up on the Eiffel Tower were listening in.

- For the French, that's intelligence gold.

They can hear what the Germans are planning

before they do it.

- [Narrator] Germany's uncoded transmissions

allowed French intelligence to forewarn their troops

and head off the German advance.

[explosions rumble]

- They were able to anticipate

where the Germans were going to allocate their troops,

where they were going to put their primary attacks,

and make sure they had forces

in place to counter those attacks.

[dramatic music] [explosions rumble]

- [Narrator] The Germans were forced to retreat

and Paris left untouched.

- This carelessness born out of speed and necessity

ends up costing the Germans many lives.

The defense of Paris was a key turning point in the war.

If the Germans had been able

to capture Paris, take France out,

the whole history of the war would've been different.

[dramatic music]

- [Narrator] The result of the disastrous offensive on Paris

is that both Germany and Britain

develop their own form of code.

Swapping letters, words, and phrases

with other words or numbers from a code book.

- The code book works like a dictionary.

You have a book with all the common words,

and if you want to encrypt the word, you need to look it up.

[suspenseful music]

[explosions rumble]

- [Narrator] Now almost two and a half years on,

coded messages are being used

by both sides for every military maneuver,

as well as top secret communications,

like the one intercepted at the Cornish relay station.

[dramatic music]

[explosions rumble]

At this stage in the war,

any intelligence on the enemy's next move

could be the difference between winning and losing.

So, as soon as the mysterious telegram arrives in Room 40,

Blinker Hall's team races to unlock its contents.

- The Allies and the central powers are exhausted.

They're running out of people

and they're running out of resources.

[explosion rumbles]

- [Narrator] As the death toll rises daily,

Britain is facing a serious problem.

There are simply not enough men

to replace the injured and fallen troops.

- The original British Forces

were well trained professional soldiers, but by 1917,

the casualties have been so high

that the majority of British troops

are going to be conscripts,

and they're gonna have relatively low levels of training.

They just don't have the experience.

World War I has become a meat grinder

and these new conscripts

are going to be thrown into the mix and ground up.

[explosion rumbles]

- [Narrator] Making matters worse, food, fuel, and weapons

are also in increasingly short supply.

- World War I is a war of logistics.

It's literally who has more material,

who can last the longest.

- [Narrator] The reliance on outside help

is becoming ever more critical.

[dramatic music]

The only thing now keeping Britain afloat

is help from America.

- America is neutral at this point,

but they are providing a vital service to Britain

by sending supplies, both of arms and ammunition,

but also of general materials and food resources.

- The most crucial transportation link

is across the vast stretches of the Atlantic,

bringing the massive material resources

of the United States,

to where the Allies need the most on the Western Front.

All the help from the United States doesn't come for free.

Britain has to pay for these goods,

either up front or by taking out loans,

and the debt is rising.

- [Narrator] American supplies

are not going to be enough to win the war.

Britain needs more men and more money.

Its only hope of survival is America joining the war,

but the US president refuses to budge.

- America at the time, sees itself as the peace broker.

This is a European conflict that it would rather see end.

They're not going to throw

their young men into that meat grinder.

[explosion rumbles]

- [Narrator] The British government

is now desperate for any intelligence

that might persuade the American president

to change his mind.

So, the pressure's on Blinker Hall's team

to deliver the goods.

Within 24 hours of getting his hands

on the encrypted telegram,

Room 40's chief codebreaker, Nigel de Grey,

has identified key information in the code.

- Although to the nonprofessional,

these intercepted messages looked like

just a jumble of meaningless numbers.

To those who are working day in, day out on these messages,

they can spot certain patterns that occur.

- [Narrator] After almost three years

of analyzing Germany's secret communications,

de Grey is able to identify

which code has been used to encrypt the original message

simply by the presence of certain number groups,

and he's got lucky with this message.

[dramatic music]

It's been encrypted with a code

already known to the Room 40 codebreakers as 0075.

This number is included at the start of the message

and is the key the receiver needs to decipher it.

0075 is the latest variant

of a code Germany has been using since the war began.

And by 1917, it's being used to encrypt

Germany's most top secret diplomatic communications.

- It's using a top priority code,

which shows that it almost portrays

that this is an important message.

- [Narrator] De Grey knows that every sequence of numbers

relates to a specific word,

but to know what that word is requires a code book.

- So, you have a plain language word,

ship, town, cloud, destination,

and for each entry of those plain language in the code book,

you then have a string of numbers.

- [Narrator] Once encrypted,

the message is then transmitted in Morse code

and sent via cable to the intended recipient,

who also has a copy of the code book.

[telegraph beeping]

- On receipt of the message,

the operator would look in a reverse code book,

see the number, look up the number,

and then find out the plain language word.

[telegraph beeping]

- [Narrator] Believing their code

to be un-crackable without a code book,

Germany now transmits

all of its most top secret diplomatic communications

using code 0075,

but its faith in its code is badly misplaced.

- All the German code books had certain similarities

and there were features which were common

to all types of code book.

So, once you started working out

how the Germans designed their code books,

then you would have insight

into a new code book when it came along.

- [Narrator] Since the war broke out,

Blinker Hall's team has managed to get hold

of several old German code books,

which are now proving invaluable.

- The Germans thought their codes were fairly secure,

but in fact, they were very, very simple codes

and they weren't changed often enough.

- Very often, the code makers used one code book

as the basis for the next one,

and so if they knew the one,

it wasn't too difficult for them to guess the next one.

[suspenseful music]

- That critical error by the Germans

now plays directly into Blinker Hall's hands.

[telegraph beeping]

The codebreakers already know the message is German

and highly top secret.

It was heading on a cable from Berlin to Washington DC.

The code used means it's a diplomatic communication

and they're about to find out exactly who sent it,

because the message's numbers hold another clue.

De Grey has discovered that the final five digit number

in every message is a signature and code,

and a search of his comprehensive records

reveals a likely match.

De Grey is certain that the telegram

has to be from top German politician Arthur Zimmermann.

- The Room 40 folks work out that this message

is from the German foreign secretary,

and the Americans had always seen Zimmermann

as someone that might be a good possibility

for brokering this diplomatic solution

to the bloodshed of the first World War.

- [Narrator] But as de Grey and his fellow codebreakers

begin to unscramble fragments of the message,

it becomes clear that America couldn't have been more wrong

about Zimmermann's ambitions.

[dramatic music]

From what they can make out,

it seems that Zimmermann

is encouraging more, not less aggression.

Their initial decoding suggests Germany is planning

to unleash deadly U-boat attacks in British waters.

And that means no vessel,

not even neutral American ships will be spared.

- Unrestricted submarine warfare means

that they can target any ship

regardless of what flag it's flying,

if they have good evidence

to believe it's carrying war material.

- The idea was to destroy the British supply lines

of ammunition, machinery and food.

[explosion rumbles]

- [Narrator] Nigel de Grey immediately realizes

how explosive this information is.

The fact that American ships

will no longer be safe in the Atlantic,

could be just what Britain needs to finally persuade the US

to come to its aid and join the war.

Because the telegram's contents

are a blatant disregard of an agreement

Germany made with the US

following a terrible military blunder.

[suspenseful music]

A year into the war,

British Navy ships formed a blockade

to stop any supplies reaching German shores.

The intention to starve Germany out of the war.

- The Germans are blockaded

and there's only a limited amount of supplies

that they have available to them.

- [Narrator] In retaliation,

The German Navy was ordered

to seek and destroy any neutral ship

that was supplying Britain with war materials.

- The Germans are gonna try to set up

their own response to the blockade.

They're gonna deploy a terrifying new weapon,

the Unterseeboot, or U-boat,

and they're going to use that, not to target warships,

but to target the vital supply lines

that are bringing in food, and fuel, and ammunition,

and the Germans realize

if they can choke off that vital supply line,

that'll be enough to end the war.

- [Narrator] But on May 7th, 1915,

off the coast of Southern Ireland,

a submerged German U-boat torpedoed

what it claimed was an enemy troop ship.

[dramatic music]

It wasn't.

[ominous music]

The Lusitania was a British ocean liner

carrying civilian passengers.

Over 1,200 innocent people died in the attack.

128 were American.

- Civilians were not supposed to be

targeted intentionally in war time,

and this is why the sinking of the Lusitania

stands out as such a big deal.

It is treated as an act of barbarism,

of brutality, of unnecessary cruelty.

- [Narrator] America was enraged

by the attack on the Lusitania,

but Germany claimed it was a case of mistaken identity

and vowed to do better.

- The Germans realize that the Americans

entering the war would be a game changer.

So, the Germans make a promise that they will redouble

their efforts to make proper identification,

to make sure any targets engaged by the U-boats

are legitimate military targets.

- [Narrator] Despite the devastating loss of life

on the Lusitania, President Woodrow Wilson

accepted Germany's new policy,

and America stayed on the sidelines of World War I.

- The sentiment in America

was very strongly against getting involved,

so this promise really gives Wilson

an excuse to keep America out.

- [Woodrow] I know that you are depending on me

to keep this nation out of war.

So far, I have done so,

and I pledge you my word that God helping me, I will.

- The public line was "Keep America Out of the War,"

supply the Allies with the equipment they need,

but don't send American boys overseas.

[explosions rumble] [somber horn music]

And they can see the casualty numbers as clearly as anybody

and to send over American men into that mess,

into what's really a European war from their perspective,

it's just not politically tenable.

[dramatic music]

- [Narrator] Two years after the sinking of the Lusitania,

President Wilson has won a second term

with his promise to stay neutral.

[boots stomping]

Britain needs some serious leverage.

If Wilson is going to perform a drastic U-turn

and send American soldiers to war.

[dramatic music]

In Room 40, even mild-mannered Nigel de Grey

believes what's been decoded so far

from the Zimmermann Telegram could prove to be crucial.

- He ran down the corridor

to Blinker Hall's office and said to him,

"Do you want to bring America into the war, sir?"

To which Blinker Hall replied, "Yes, why?"

Well, we've got the telegram that might do it.

- For those who do want to see

America get off the sidelines,

this could be the thing that makes it happen.

[suspenseful music]

- [Narrator] But there's a problem.

In fact, there are several.

- The first part is, it's only been partially decoded,

so there's a huge risk moving forward with this information

before you have all the information available.

- [Narrator] The second issue is security.

If even partial contents are released,

the Germans will know the British have cracked their code.

- If they reveal that they can read any of that code,

the Germans will immediately change their system overnight,

and all of the British decoding work

done in Room 40 will be wasted.

They'll have to start from scratch again.

[dramatic music]

- [Narrator] But the third issue is the hardest to overcome.

It's the way the message was intercepted in the first place.

- The British have got this information

by spying on the Americans.

[telegraph beeping]

[suspenseful music]

- [Narrator] The root of the problem

dates back two and a half years to July, 1914,

just hours after World War I was declared.

In a covert operation,

a cable laying ship slipped out of the port of Dover

on Britain's south coast.

It made its way to the middle of the English channel,

weighing anchor directly above

a series of German owned telegram cables,

connecting Germany to America under the Atlantic Ocean.

The crew of the CS Alert then began

systematically tearing up every cable from the seabed.

- So the transatlantic cables

are one of the great feats

of communications technology in that period.

There are five German cables going across the Atlantic

and the CS Alert severs all of them.

- [Narrator] For Britain, this was a tactical master stroke.

For Germany, a monumental disaster.

- All German communications internationally were cut off.

- The modern equivalent would be, as soon as war breaks out,

shooting down the enemy satellites.

You are blinding them and deafening them.

- [Narrator] With its vital lines

of communication out of action,

Germany had no choice but to seek outside help.

- If they want to communicate

with their representatives in Washington,

they're going to have to use the cables

that belong to another country.

- [Narrator] By 1917,

any telegram sent between Germany and America

must travel along the cable route

that passes through British waters,

[telegraph beeping]

and wireless operators at the remote telegram station

on the Cornish coast are secretly tapping the line,

intercepting every message that uses the underwater cable

linking Britain to America.

And that's why intercepting the Zimmermann telegram

is such a big deal.

[suspenseful music]

The message the British codebreakers hope

will force America into the war

has been picked up by eavesdropping

on supposedly secure lines

reserved for highly confidential diplomatic traffic.

- America had agreed to let the Germans

continue to use these cables

specifically for peace negotiations.

But of course, that wasn't entirely what they were used for.

- Britain had been decoding American messages

from late 1915,

and so were keeping an eye on what was going on

and rapidly saw the German messages

being passed bundled in with American messages.

- [Narrator] Germany may have been

abusing America's goodwill,

but Britain has been snooping on its potential ally.

- So, the British have to be delicate about this.

The last thing they want is for the Americans

to think that they are being spied on.

[dramatic music]

- [Narrator] Even though Blinker Hall's team

has decoded enough of the Zimmermann telegram

to make the case that Germany

is about to start targeting

American merchant ships in the Atlantic,

they need a rock solid cover story to explain how they know.

- The British are in possession

of an essential piece of intelligence.

But if they're not extremely cautious in how they use it,

they run the risk of offending someone

they're seeking as an ally.

- [Narrator] Blinker Hall knows now is the time

to refer up and share Room 40's findings with his boss,

the British Prime Minister, David Lloyd George.

Despite Britain desperately needing help,

Lloyd George believes the timing isn't right,

revealing the secretly intercepted message to America now

could be catastrophic for future relations.

So, he decides the only thing to do is wait.

- If he just sits on this

and Germany goes through with what they're threatening

unrestricted submarine warfare,

that might bring America into the war,

and they have good reason to believe that.

[dramatic music] [explosion rumbles]

- [Narrator] On February 1st, 1917,

two weeks after the telegram was intercepted,

Germany launches the offensive

Zimmermann outlined in his coded telegram.

Unrestricted U-boat warfare in the Atlantic

is a clear betrayal of the agreement

made between Germany and America,

but will this aggression

be enough to bring America into the war?

[dramatic music]

In Washington, D.C.,

President Wilson's reaction to Germany's hostile actions

is a long way

from what the British government is hoping for.

- And though America breaks off

diplomatic relations in protest,

they don't immediately enter on the side of the Allies.

- [Narrator] The wait and see approach has backfired,

but there's still one last chance

to change the US president's mind.

- The unrestricted submarine warfare

didn't bring America in.

But what about the other part of the message?

[suspenseful music]

- [Narrator] Back in London,

Blinker Hall's team is now working day and night

to decode the rest of the message

in the hope it contains more information

to bring America into the war.

As Nigel de Gray unpicks more and more fragments,

he discovers something unexpected.

Although the telegram was heading for America,

the US wasn't the telegram's final destination.

After reaching Washington D.C.,

the Zimmermann telegram was forwarded on to Mexico.

And this immediately raises a red flag.

- Germany was making diplomatic overtures to Mexico,

which appeared to be trying to entice them

into joining the war on German side.

- [Narrator] And the more information the team unscrambles,

the more shocking the intelligence becomes.

- They can only understand patches of that code,

and yet the potential is massive.

- [Narrator] They want Mexico

to go to war against the United States of America.

- This has the potential

to change the entire course of the First World War.

- [Narrator] The German foreign secretary

is a hard line strategist,

and he's desperate to see Germany win the war.

He hopes that exploiting Mexico's lingering anger

with the US over territory the country lost

to America 70 years earlier will do just that.

[suspenseful music]

- There's no small number of people in Mexico

that still see that territory as Mexican territory,

not American territory.

- [Narrator] Zimmermann appears to be promising

that Germany will finance a military invasion of the US,

allowing Mexico to claim back

their lost states of Texas, Arizona, and New Mexico.

- It is astounding that the German foreign secretary

would write to the Mexican authorities,

encouraging them to make war on the United States of America

and to claim territories such as Texas

from the United States of America,

and yet this is precisely what this telegram does.

[suspenseful music]

- If Blinker Hall and his team's decoding is correct,

Zimmermann's plan is a clever one.

A war on home soil will distract America,

effectively keeping the country out of World War I for good.

- In effect,

this cable was supporting a direct attack on American land.

- [Narrator] An American Mexican war

would also have the added benefit

that the US supplies keeping Britain in the war

would dry up.

- It would force America to redirect all the supplies

it's been sending across the Atlantic

to the Allied war effort in Europe,

to defending its own borders.

- [Narrator] Increased aggression by U-boats

may not have been enough

to change President Wilson's mind,

but Blinker Hall and his team are hoping

that the very real threat of a border war with Mexico

will finally hit home.

[dramatic music]

[explosions rumble]

But as with the previous intelligence,

they're still facing the same problem

of how the information was gathered in the first place,

by British intelligence,

spying on America's supposedly secure

diplomatic communications.

[telegraph beeping]

- Blinker Hall had to work out

how to deliver this information,

covering up what he needed to cover up,

but exposing what he needed to expose.

[suspenseful music]

- [Narrator] Three weeks after intercepting

and decoding the telegram's incendiary contents,

Blinker Hall comes up

with a brilliant solution to the problem.

It's complex,

but it could provide the perfect cover story,

if he can pull it off.

The Zimmermann telegram left Berlin

and was sent onto Copenhagen,

then Britain by overland and underwater cables.

[telegraph beeping]

It then reached Washington D.C.

via transatlantic cable, but crucially,

it would've been relayed from Washington D.C.

to a telegram station in Mexico city via a separate cable,

and that means a second copy of the message must exist.

If it does, it will be a version of the telegram

that couldn't have been picked up

by British telegram operators

illegally tapping the US cable lines.

- If they can get hold of that second message,

that would be much more useful,

at least in terms of publicly revealing how they got it.

- [Narrator] In Mexico,

a British agent known only as Mr. H,

is tasked with locating the relay telegram.

- It turns out that the interception

of the second message is much easier.

They do it the old-fashioned way using money.

- By bribing some post office officials,

they managed to get this original second cable.

- [Narrator] Blinker Hall's inspired plan

has worked to perfection,

but he's about to be thrown a curve ball

that no one in Room 40 is expecting.

[telegraph beeping]

[dramatic music]

Within 48 hours of the Mexican operation,

Blinker Hall has two copies of the Zimmermann telegram,

but it's immediately clear to the Room 40 codebreakers

that the new Mexican copy

is not identical to the original intercepted message.

- It turns out that that second cable was an earlier code.

- So, now we have another decryption task.

[suspenseful music]

- [Narrator] Once again, luck is on their side.

As with the first telegram,

a string of numbers at the start

of the Mexican message provides a clue.

The number 13042 tells the receiver which code

has been used to encrypt the message.

[suspenseful music]

This code is a close relation

to an earlier diplomatic cipher

that Nigel de Grey and his team are not only familiar with,

they've already cracked it.

[dramatic music]

- So, rather fantastically,

the British can now read the entire message,

not just patches of it.

- [Narrator] Within a matter of hours,

the team has unscrambled the full explosive contents

of Zimmermann's message to the German ambassador in Mexico.

Even better, it confirms the codebreaker's decryption

of the original message.

[telegraph beeping]

First, the U-boat offensive in the north Atlantic.

- [Codebreaker] The intent to begin on the 1st of February

unrestricted submarine warfare.

- The second message confirms

everything they have from the first message.

This is an explosive piece of intelligence.

- [Narrator] Then, as already suspected,

Zimmermann's plan is to forge a military alliance

with Mexico to keep America's focus

on home soil, not Europe.

[gunfire blasting] [explosion rumbles]

- [Codebreaker] We make war together, make peace together.

Mexico is to reconquer the lost territory

in Texas, New Mexico, and Arizona.

- You even get a little extra from it,

which is an invitation to the Mexican president

to encourage the Japanese to also join the war

on the side of the central powers.

[dramatic music]

- [Codebreaker] Signed, Zimmermann.

- Room 40 has found the very piece of evidence that it needs

to lead the United States of America

into war against Germany.

- [Narrator] Blinker Hall's plan has worked.

The new fully deciphered version of the telegram

leaves nothing to the imagination

about Germany's planned acts of aggression against America

and even better,

British intelligence now has a cast iron cover story

for how the top secret information was gathered.

[suspenseful music]

- This is really the perfect outcome for the British.

They can use this essential piece of intelligence

the way they wanted to use it,

but they can do it without revealing

that they've broken this German code

without revealing that they've been spying

on the United States for years already.

- [Narrator] On February 24th, 1917,

four weeks after the original message was intercepted,

the copy of the telegram sourced from Mexico

is presented to President Wilson.

- Wilson's reaction to this explosive telegram

that threatens a war with Mexico,

that threatens to bring in Japan is outrage.

- "Oh Lord," he says, and he knows immediately

that American neutrality can be no more.

They cannot accept a planned invasion

or attack on American territory.

[dramatic music]

- [Narrator] Four days later,

the Zimmermann telegram is published in the American press.

US intelligence is credited with intercepting

and deciphering the scandalous message,

keeping attention away from Blinker Hall's very British team

of codebreakers in Room 40.

- This telegram is particularly startling

because it brings the war home to their own borders,

a border with a nation such as Mexico,

who they already have conflict with.

It makes Americans feel that they're not safe.

[dramatic music]

- [Narrator] Incredibly,

the telegram's contents are so shocking

that doubt begins to surface about its authenticity.

- Although the Americans have said

that it was American intelligence that found this telegram,

there is still a number of suspicions around it.

- [Narrator] Proof that the information is correct

comes from an unlikely source.

- Zimmermann himself puts his hand up

and admits that he wrote this telegram,

so authenticity is certified.

- The American public react

with utter shock at this revelation.

- [Narrator] In Washington,

President Wilson knows there's no ignoring this information.

[dramatic music]

As well as the threat of a German backed war with Mexico,

he's also now facing the stark reality

of the increased U-boat aggression

he had previously chosen to ignore.

[explosion rumbles]

- In March, 1917,

the gloves have really come off in the Atlantic.

The Germans are now torpedoing any ship that they suspect

are carrying war material of any sort.

At this point, Wilson really has no choice.

He has to act decisively.

[dramatic music]

- On April 6th, 1917,

three months after the Zimmermann telegram

was first intercepted,

President Wilson finally abandons

his long held policy of neutrality,

and America officially declares war on Germany.

[dramatic music]

- This is really the worst possible outcome for the Germans.

The Americans can now release all the safety catches,

flood the Western Front with material,

and worse yet they can bolster the depleted ranks

of the Allied troops with this immense pool of manpower

that they can draw on.

[dramatic music]

- [Narrator] By May, 1917,

the American expeditionary forces have arrived in Europe.

Within weeks,

the war has begun to turn in favor of the Allies.

[explosions rumble]

- Prior to this point,

Germany had every reason to believe

they'd at least be able to fight the Allies to a stalemate,

but now the Americans have come in,

and it's a whole new ballgame.

The German leaders and the German generals

were willing to keep on fighting,

but the Germans themselves had had enough.

They are not willing to fight anymore.

[people cheering]

- [Narrator] On the 11th hour, of the 11th day,

of the 11th month of 1918,

World War I is officially over.

[triumphant music] [people cheering]

An estimated 16 million people have been killed

in the so-called war to end all wars,

and a further 21 million injured.

[dramatic music]

The decoding of the Zimmermann telegram

was a crucial turning point in World War I,

yet the incredible contribution

of the former Navy captain Sir Reginald "Blinker" Hall

and his pioneering team of codebreakers

remains a closely guarded secret for decades to come.

- So, why don't we hear more

about the extraordinary efforts of Room 40

and the key role they played in bringing America

into the war and thus allowing for an Allied victory?

[dramatic music]

Well, secret codebreakers are only good

as long as you keep 'em secret.

[suspenseful music]

- [Narrator] After the war, Blinker Hall retires,

but keeps active as a lecturer on intelligence gathering.

Nigel de Grey stays with Room 40

as the team is merged with the Army's intelligence agency.

The new outfit becomes

the world famous Government Code and Cipher School,

which has moved 50 miles north

of the Admiralty building to Bletchley Park.

- So we all know about Bletchley Park and the enigma machine

and how crucial that was

to the Allied efforts in the Second World War,

but Room 40 was really the origins of all that.

- [Narrator] The contribution

of Bletchley Park's codebreakers in World War II

can never be underestimated,

but it may never have happened

without the groundbreaking work of the Room 40 team.

- The breaking of the Zimmermann code

illustrates how important cryptography is to the war effort.

- The Zimmermann telegram

is probably the most influential coded telegram message

sent certainly in the First World War,

and possibly in both world wars.

- This was one telegram that changed

not just the course of the war,

but the entire course of modern history.

[dramatic music]