All language subtitles for Cracking.The.Code.S01E07.720p.AMZN.WEB-DL.DDP2.0.H.264-Kitsune.[eng]

- [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]