Original subtitles

[bomb exploding] [dramatic music]

- [Narrator] World War II's war in the Pacific.

The battle for supremacy will depend

on unraveling unbreakable codes and ciphers.

- When they saw the five number groups,

they were like, what is this?

- You don't know what it means.

And you've gotta find a way of breaking it.

[propellors whirring]

- [Narrator] Two brilliant minds go head to head.

Japan's most formidable Naval strategist.

- Yamamoto's plan was to destroy the American Pacific fleet.

- [Narrator] And a U.S. officer

with a penchant for crosswords.

- He was such a whiz at puzzles,

crossword puzzles, number puzzles.

He could almost see them intuitively.

- [Narrator] But can codebreakers really stop

the Japanese onslaught

and turn the war in America's favor?

- They need to crack JN-25.

[tense music]

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

Bizarre markings, random letters and numbers,

words that make no sense.

But cracking them unlocks military secrets,

decodes ancient civilizations,

[gun firing]

and reveals enemies in our midst.

Now we uncover how they were decoded,

the genius minds that broke them,

and the secrets they revealed.

[light music]

December 7th, 1941.

The Pacific islands of Hawaii.

[electronic beeping] [ominous music]

Just after dawn, the tranquility is shattered

as Japan launches an all out attack

on the U.S. Naval base at Pearl Harbor.

- Seven o'clock in the morning, planes appeared overhead.

[propellors whirring]

Then the bombs exploded.

[bombs exploding]

[suspenseful music]

- The Japanese planes sweep across the island,

strafing and bombing at will.

- There were eight battleships in Pearl Harbor

on that morning.

Four were sunk outright, four others badly damaged.

More than 2,400 Americans were killed

and many thousands more wounded.

[ominous music]

- [Narrator] The man behind the surprise attack

is Japan's formidable Admiral Yamamoto.

His aim, to clear the way for Japan's Imperial expansion

throughout the Pacific region.

- The attack on Pearl Harbor

was not to conquer the United States.

It was to knock the American fleet out of the way

so they could gain access to the resources

in the South Pacific.

- Japan is a heavily industrialized nation,

but it's short on raw materials.

So by seizing these territories,

Japan is going to gain the resources it needs

to build its military, to build its industrial strength

in a way that would rival that of even the greatest

of European nations.

[dramatic music]

- [Narrator] Four days after the lethal strike,

America joins the war in Europe.

- This was completely unexpected from the American side.

It was absolutely devastating.

This completely changed the American mindset on the war.

- [Narrator] But on home soil,

concern is growing that the attack

that crippled the U.S. Navy is just the beginning.

Harvard graduate and master poker player Yamamoto

knows his enemy and the value of playing his cards

close to his chest.

- Might alone was not going to defeat the Japanese.

Cryptographers working in dusty offices

were gonna be just as crucial to victory in the Pacific

as men and ships would be.

[propellors whirring]

- [Narrator] To make sure his enemies stay in the dark,

Yamamoto has ensured all military communication

is heavily encoded,

not once, but twice,

using a complex and top secret code

known to Japan's enemies as JN-25.

- All planning and movement of operations

were done via JN-25.

That was the main operational code.

Everybody had it.

- [Narrator] For months,

Allied intelligence has been intercepting

Japanese JN-25 messages,

[electronic beeping]

but cracking the complex super code

has so far proved impossible.

- The British had a base in Singapore,

and they're listening into the Japanese signals,

but they can't understand them.

- The Japanese Imperial Navy thought JN-25

was utterly unbreakable.

[suspenseful music]

[cannon firing]

- [Narrator] Already on the back foot

following Pearl Harbor,

unlocking Japan's secret code is the only chance America has

of finding out what Admiral Yamamoto's next move will be.

- They need to crack JN-25.

They need the crucial strategic advantage

that being able to read the Japanese coded signals

will give them.

[upbeat music]

- [Narrator] The seemingly impossible task of breaking JN-25

begins in a smoky, windowless basement

at the Naval base in Pearl Harbor, known as the Dungeon.

This is the nerve center for the U.S. Navy

Signals Monitoring and Cryptographic Intelligence Unit,

or Station Hypo.

Heading up the crack American code breaking team

is Joseph Rochefort,

a Naval officer with a penchant for crosswords.

- Now Rochefort was not a Naval Academy graduate,

which perhaps to some made a difference,

but he had been recommended for the code section

by a commanding officer early in his career

because of his innate ability to solve puzzles.

He was such a whiz at puzzles.

He could almost see them intuitively.

- [Narrator] Rochefort isn't just a crossword genius.

He has a second killer skill.

He's a gifted linguist and has a passion for Japan,

spending years studying the country's language and culture.

[energetic music]

- His cultural understanding of Japanese society

allowed him to see things in these messages

that few others might have discerned.

- [Narrator] But in December, 1941,

Rochefort is facing one of the toughest challenges

of his Navy career.

He has hand picked the Hypo team,

not only for their codebreaking skills,

but because, for all of them, cracking JN-25 is personal.

- The cryptoanalysts who worked

in those air-conditioned basement rooms at Station Hypo

were driven, by a sense,

to break the Japanese operational code.

They knew that if they had done so prior to December 7th,

it might have been possible for them to give warning

of the pending Japanese attack.

And they Flad themselves almost daily

for the fact that they had been unable to do. So.

- They'd been in the thick of it, so to speak,

because you know, they were in Hawaii.

They had suffered friends had died.

[dramatic music]

- They work literally around the clock trying to find pieces

of information that would allow them to warn their bosses

of the next Japanese initiative.

[soft music]

- [Narrator] It's a monumental challenge.

But thankfully, Rochefort and his team

aren't starting completely from scratch.

For the past two years, British codebreakers,

based at the Government Code and Cipher School

at Bletchley Park just outside London

have been intercepting Japanese Naval communications.

The result is pages and pages of apparently random sequences

of five digit numbers.

[electronic beeping]

- When they saw the five number groups,

they were like, what is this?

That's the hardest thing in cryptoanalysis

is solving the system.

When you're faced with nothing but pages

of five digit numeral groups.

[tense music]

- [Narrator] Hunting for a way to understand

how the encryption worked,

British codebreakers began the mammoth task

of gathering and analyzing every message

that had been intercepted.

- The first requisite is to accumulate enough traffic,

enough messages, where you can start to see patterns.

Am I looking for repeats and groups

that tells me that somehow these things are related.

- [Narrator] Getting hold of the JN-25 messages

was the easy part.

- All navies use radio transmissions

to communicate with their fleets at sea.

Radio messages sent by the Imperial High Command in Tokyo

out to the fleet were sent by telegraph messages

in Morse code.

- [Narrator] What was proving almost impossible

was working out what the Japanese were sending

in Morse code.

The internationally recognized communication system

works by breaking up words into their individual letters.

But the Japanese language doesn't use letters.

It uses logograms and characters,

unique images and symbols that represent words and syllables

rather than letters.

Because there are no letters,

in theory, Morse code shouldn't work.

Yet thousands of messages were being sent and received

by the Japanese Navy.

So how did JN-25 work?

The first breakthrough came from Britain,

and one of Bletchley Park's most extraordinary codebreakers.

- The first person to make inroads into the JN-25 code

was Brigadier John Tilman.

He was a Scottish officer.

He was known as the Brig,

and he wore his army uniform with tartan trousers.

He was quite a character.

He was also absolutely brilliant.

[light music]

- [Narrator] Tiltman was convinced

that each five digit number stood for a different word,

but he discovered no message ever contained

the same sequence of numbers twice.

Tiltman realized that the fact there weren't any repetitions

meant JN-25 must contain a second layer of encryption.

- Tiltman works out it's been additionally scrambled

with what called additive numbers.

So groups of extra numbers just put in

to confuse any codebreaking attempts.

- [Narrator] That meant decoding any message sent as JN-25

needed two books,

each one containing a different set of numbers.

- One is a code book, which translates words into numbers.

And one is a cipher book,

which is full of streams of random numbers.

- [Narrator] Creating a code in JN-25

was a two step process.

Firstly, the sender would use the code book

to replace every word

with a corresponding five digit number.

Then they would use the cipher book to choose another

sequence of five digit numbers,

this time completely random ones.

The random numbers would then be added

to the original number to superencipher the code.

And it's these numbers that are then sent

by Japanese Morse code.

Although Tiltman had discovered how the code worked

two years before the attack on Pearl Harbor,

without a Japanese cipher book

to at least unscramble the first layer of encryption,

it was impossible to make sense of the messages.

[tense music]

In Hawaii in 1941,

John Tiltman's discovery has given Joseph Rochefort

and his team a small head start in the race to crack JN-25.

The other advantage the U.S. codebreakers

have over their British counterparts

is the sheer amount of data now available to them,

because a surge in Japanese military activity

means JN-25 coded messages are now pouring in.

- The Japanese were running almost rampant

in the Western Pacific.

[soldiers shouting]

- [Narrator] The fear is that Admiral Yamamoto

is gearing up for another big attack.

- Another devastating unexpected loss

on the part of the U.S. is very real.

- The Japanese were gobbling up those resource bases

in the South Pacific,

and there was great fear in America that they might extend

this even further,

that Australia might in fact become a target,

that Hawaii itself might be targeted.

[suspenseful music] [gun firing]

- [Narrator] The Japanese advance

is beginning to look unstoppable.

The clock is ticking and the pressure is on Rochefort

and is small team of codebreakers

to find out what Yamamoto is planning next.

[light music]

By early 1942, the team in Hawaii is stuck.

They know how JN-25 works

and they have thousands of messages to analyze.

But without the Japanese Navy cipher book

to unlock the super code,

all they have are random strings of numbers.

- You don't have the code book.

You don't have the cipher book.

You're starting with nothing.

- [Narrator] In desperation.

Rochefort decides to try to reverse engineer the code.

[energetic music]

It makes sense the person receiving the message

will need to know how to first decrypt,

then decode the numbers.

[electronic beeping]

The codebreakers figure there must be a clue in each message

guiding the receiver to the correct numbers to start with.

But with so many messages to analyze,

it's not humanly possible to identify

which five digit number is the all-important key.

- The Japanese saw how complicated this was

and believed it all but impossible for any eavesdropper

to break that code.

[cannon firing] [dramatic music]

- While Rochefort and his team battled to unlock JN-25,

the Japanese offensive in the Pacific

continues to be relentless.

In the weeks since the attack on Pearl Harbor,

Hong Kong and the American controlled Philippines

have been invaded.

Singapore has fallen,

and the Japanese have taken the U.S. Pacific Islands

of Wake and Guam.

The daunting result of this aggressive offensive

is yet more encrypted messages for Rochefort and his team

to work with.

- After the operations on 7 December, 1941,

the volume of radio traffic increases dramatically,

probably three, fourfold.

- The number of messages sent down into the Dungeon

for analysis was staggering.

At least 500, sometimes a thousand per day.

- [Narrator] The deluge of data means Rochefort's small team

is now having to work around the clock.

In desperation,

they turn to technology to help process the information.

It's a decision that will give them their first major break.

- The card sorting machines were designed to find patterns

in the Japanese messages.

They didn't really try to solve a problem

the way a computer would solve a problem,

but they were a shortcut for the cryptoanalysts

in trying to discern patterns or repetitions.

- [Narrator] Using the sorting machine is a game changer.

Rochefort's team begins to spot the same sequence

of numbers appearing time and again.

They suspect it's the code that pinpoints

the start of the column of numbers in the cipher book

used in the second level of encryption.

Even better, it seems the people coding the messages

are creatures of habit.

- Hypo starts to see a tendency

among Japanese radio operators

to begin their use of additives in the same spot

on the same page very often.

For example, they would go to the upper left hand corner

and begin with that over and over again.

- The same sequence of numbers from the cipher book

is being used repeatedly to encrypt the numbers

from the code book.

[electronic beeping]

But frustratingly,

because they don't have the all important code book

that converts numbers into words,

they can only guess at the original content of the messages.

- You know what sort of things might be there,

but you don't know what's in that message.

[propellors whirring]

[dramatic music]

- [Narrator] By late January, 1942,

the Japanese have taken Rabaul,

a vital natural harbor in New Guinea.

The Australian mainland is now under a very real threat

of attack.

But Rochefort's codebreakers are still a long way

from cracking JN-25

and delivering the much needed intelligence

that could stop the Japanese offensive.

All they've got is a set of numbers

that means nothing without the right code book

to decode them.

- The JN-25 code book had 33,000

different possible entries in it.

Putting that back together was an immense task.

- It's now that Joseph Rochefort,

linguist, crossword enthusiast,

and lover of Japanese language and culture

comes into his own.

- The Japanese had a very formal approach

to sending messages, and very, very respectful.

[dramatic music]

- [Narrator] Rochefort reckons this formal way

of communicating means the same words and phrases

will be used time and again

in all naval communications.

[electronic beeping]

These presumed words, also known as cribs,

can then be used as a way of starting

the decryption process.

Using Rochefort's intimate knowledge

of the Japanese language,

the codebreakers begin to tentatively replace numbers

with words for the first time.

- John Rochefort could look at set of code groups

and see some words and he said, well,

this is what you would fit in,

here because it's a naval context.

- If you can work out what the context is,

then you can begin to guess the content.

And then you begin to rebuild the code book.

- [Narrator] After weeks of painfully slow analysis

and guesswork, Rochefort and his team

are beginning to achieve the impossible.

[ominous music] [soldiers shouting]

But they're struggling to keep up with a Japanese onslaught.

By March, 1942,

the Japanese are advancing relentlessly through Burma,

and Darwin in Northern Australia

has suffered a major airstrike.

As mainland Australia begins to look

under serious threat of invasion,

Rochefort scores his first significant hit.

- In April, Hypo partially decodes a message

indicating the Japanese have dispatched a carrier group

to the South Pacific.

And a British intercept reveals the Japanese

are moving two fleet carriers.

- [Narrator] The messages reveal elements

of Admiral Yamamoto's next step

in his plan to conquer the entire Pacific region.

He's calling it Operation MO,

and Rochefort's increasing understanding of the super code

provides his team with a possible target.

Rochefort is convinced Operation MO

is the Japanese code name for Port Moresby,

an allied controlled base north of Australia on New Guinea.

- The Japanese used a two letter indicator

for all geographical locations.

Then Rochefort kept a careful record

of these geographical locators.

- [Narrator] Yamamoto's intentions are immediately clear.

Port Moresby is the perfect launchpad

for a Japanese invasion of Northeastern Australia.

- Port Moresby is a seaport

on the southern edge of new Guinea.

- It has an airstrip.

It has a repair station.

It has sub docking facilities.

So it holds crucial strategic importance.

- [Narrator] Taking the poorly equipped garrison

at Port Moresby should be a cakewalk

for the might of the Imperial Navy.

But Yamamoto has no idea that Rochefort

and his team of Codebreakers are onto him.

- The Americans had found a way

to break into those various layers of secrecy

and figure out what was inside those messages.

[dramatic music]

- [Narrator] On the basis of the intelligence

provided by Rochefort's team,

Admiral Nimitz, Commander in Chief of the U.S. Pacific fleet

dispatches a battle group of 26 vessels,

including the aircraft carriers Lexington and Yorktown

towards Port Moresby in the Coral Sea.

- It was a significant risk for Nimitz to say,

I'm gonna send my two carriers into the Coral Sea

to stop the Japanese.

And he wouldn't have done it at all

but for the codebreaking effort of Station Hypo.

- [Narrator] On May 3rd, 1942,

Yamamoto orders the start of the operation

to take Port Moresby.

[propellors whirring] [suspenseful music]

But the Americans are waiting for him.

- The Battle of the Coral Sea

is a really crucial naval engagement.

It's a carrier on carrier battle

fought largely by the planes being launched

from those carriers.

[propellors whirring]

- Each carrier force sends an airstrike to attack the other,

170 miles away roughly.

So the two fleets never actually see one another.

[bombs exploding]

- [Narrator] During the two day battle,

Japanese torpedoes and bombs

cripple the Lexington beyond repair,

and they drop an armor piercing bomb

deep in the Yorktown.

- The Japanese pilots came back reporting

that they had sunk not only the Lexington,

but also the Yorktown.

And the Yorktown was badly damaged,

but it was not sunk.

It continued to be operational,

and was able to withdraw under its own power

back to Pearl Harbor.

- [Narrator] The U.S. may have taken a hit,

but Yamamoto's Imperial Navy doesn't get off scott free.

[plane exploding]

He also loses a carrier.

And the fleet carrier Shokaku is so badly damaged,

it's forced to limp away from the battle.

[soft music]

Both sides retire to lick their wounds.

But crucially, Port Moresby is safe, for now.

- Technically it's a stalemate,

since both sides are forced to withdraw

and neither can claim decisive victory.

But it's kind of a morale victory for the Americans

'cause it's the first time they're able

to successfully resist an incursion by the Japanese.

[somber music]

- [Narrator] The U.S. Navy was only able to stop the attack

on Port Moresby thanks to the codebreaking efforts

of Rochefort's team back on Hawaii.

- This is the first moment that the Japanese

were prevented from doing something that they wanted to do.

It's the turning point in the Pacific War.

- [Narrator] Although Rochefort and his team

are directly responsible for slowing down Yamamoto's plans,

trouble is looming.

[tense music]

In Washington, DC.

the new head of Naval intelligence, John Redman,

believes Rochefort is a loose cannon

who ignores the chain of command.

- Joe Rochefort was supposed to send his information

to John Redman,

who would then come up with operational suggestions

for Nimitz.

Instead of that,

Rochefort went straight to Nimitz's headquarters.

And that left Redman out of the loop.

- [Narrator] Rochefort's unorthodox approach

is making enemies at home that will come back to haunt him.

But right now he has a bigger problem on his hands.

[foreboding music]

His team has intercepted JN-25 messages

which seem to indicate Yamamoto is gearing up again.

- As the Battle of the Coral Sea was being fought,

Hypo, back in Hawaii, was beginning to get messages

that suggested there was going to be another

Japanese offensive, an even bigger Japanese offensive.

- [Narrator] Rochefort's team has spotted

another two letter location code

cropping up in Japanese Navy messages

with increasing regularity.

- They begin to see the references to AF,

that this engineering unit is going to go to AF,

that this seaplane reconnaissance unit

is going to be transferred to AF.

And so they begin to see this and they say, well,

where is this AF going to be?

- [Narrator] Worse, from the sections of the messages

Rochefort and his team are now able to decode,

there is the suggestion that Admiral Yamamoto

is planning to hit this mystery location

with one of the most deadly naval assaults in history.

The problem is that AF could be almost anywhere

in the Pacific.

- Is it going to be another attack on Pearl Harbor?

Or perhaps even an assault on the American west coast?

Are they gonna go for the Aleutian Islands in Alaska?

There's a lot of ocean and a lot of territory.

- [Narrator] But Rochefort knows he's seen AF before.

- For months before they even found it

in the message traffic,

he had a list of geographical designators,

MO for Port Moresby, RR for Rabaul,

so that when he found one in a code,

he could understand probably what it referred to.

- Rochefort is convinced Yamamoto's next target, AF,

Is Midway Atoll.

[dramatic music]

Two tiny islands in the middle of the Pacific ocean.

- On the map, it doesn't look like much,

tiny fly speck in the middle of the vast Pacific ocean.

But there's some crucial facilities there.

- Those two islands post an airfield and an anchorage.

So it had a certain operational value.

So it is, as its name suggests,

kind of a midway stopping point for transpacific airlines.

- [Narrator] Rochefort immediately sees how Midway Atoll

could be of strategic interest

to the Japanese Imperial Navy.

Despite previous warnings from his new boss in Washington,

Rochefort once again ignores the chain of command

and goes straight to the top.

- Rochefort reported to Nimitz

that the Japanese were planning a major offensive

against Midway Atoll sometime toward the end of May

or the beginning of June, that it would have four,

possibly five carriers plus battleships.

This was going to be a major effort.

- [Narrator] Rochefort's prediction is once again the result

of JN-25 decrypts, radio traffic data,

ship movements, and his own gut instinct.

But in Washington, Rochefort's new boss John Redman

is far from convinced that AF is Midway Atoll.

- There was concern that the Japanese

might try something else.

They might retry to take Port Moresby.

They might try a second attack on Pearl Harbor.

There were some who even suspected that the target

might be the west coast of the United States,

or even the Panama Canal.

- [Narrator] Redman thinks that acting on Rochefort's hunch

is too big a gamble for the U.S.

If he's wrong,

diverting ships and manpower to the middle of the Pacific

will leave other key U.S. sites wide open to attack.

- Rochefort is thinking, hey, Midway is the target.

It's that simple.

And Redmond says, well,

you don't have enough evidence to convince me

that Midway is the target.

The Japanese could be deceiving us.

- [Narrator] America cannot afford

to get this decision wrong.

The clock is ticking on another major Japanese attack,

and neither Rochefort nor Redman will back down.

Then one of Rochefort's team has a light bulb moment,

a bluff to flush out more information.

- Why don't we send a message to Midway

using the submarine cable,

so that it's not intercepted by the Japanese,

to have Midway tell us by radio

that their salt water evaporator has broken down

and that they're short of fresh drinking water.

- [Narrator] If Japan is planning on taking over Midway,

they'll know a working desalination plant

to produce drinking water from seawater is crucial.

Without it,

the base would be unable to support an invading force.

- If you're gonna station a Japanese force on Midway

and you don't have a desalinization machine,

those people are all going to die of thirst.

- [Narrator] If AF is Midway,

Rochefort knows Japanese intelligence

will immediately understand the problem

and will begin sending out encrypted messages

warning Yamamoto's fleet about the broken water plant.

And if that happens,

then intercepting and decoding those messages

will confirm Rochefort's theory.

It's a devious plan, but Admiral Nimitz likes it.

- The message goes out into the air.

Then he waits.

[suspenseful music]

- [Narrator] It's 24 hours before a response is picked up.

[dramatic music]

- Sure enough, the Japanese intercept that radio message

and report back to Tokyo

that AF

is short of water.

There you are, smoking gun.

That proves to the skeptics that AF is indeed Midway.

- [Narrator] It's vindication for Rochefort.

But more importantly,

the decrypted messages reveal yet more

of Admiral Yamamoto's master plan

for control of the Pacific.

Midway isn't the main target.

It's a stepping stone to a far bigger goal.

- Yamamoto's plan wasn't so much to seize Midway,

although that was part of the operation.

His main objective was to destroy

the American Pacific fleet.

And specifically the aircraft carriers.

- Yamamoto wanted to get those carriers.

He never felt that he could really control

the Western Pacific until he could put those carriers

on the bottom of the ocean.

- [Narrator] Yamamoto is gambling that Midway Atoll

is too important for America to lose.

And once he's taken it,

he's banking on a swift retaliation from the U.S. Navy.

- Midway had an anchorage, it had an airfield.

It was a stopping off point for American submarines

on their way to the Western Pacific.

And Yamamoto hoped that the Americans

would consider that it had enough strategic value

to warrant their sending out their carriers to fight for it.

- [Narrator] Yamamotos plan is to ambush

the cream of the U.S. Navy,

taking it out of action for good.

But its success relies on total surprise.

He has no idea that Rochefort's team

has begun to unravel JN-25.

They are now able to decrypt up to 30% of every message.

Slowly but surely,

more and more words from the Japanese code book

are being revealed.

But Rochefort and his team can't afford to rest.

- Some of those who receive the radio messages

sometimes work 12 hours on, 12 hours off.

The cryptoanalyst down in the Dungeon

worked even longer hours than that.

And if they were in the middle of a message

that looked promising, they might just continue working.

- [Narrator] Rochefort's intelligence allows the U.S. Navy

to prepare for Yamamoto's surprise invasion.

But the codebreakers know the inroads they've made

could disappear in a heartbeat

if the Imperial Navy decides to change the JN-25 code book.

- If the new code book came in,

they would've had to start from nothing again.

- [Narrator] But Rochefort's team

is about to catch a lucky break.

The Japanese are about to become victims

of their own successful battle plan.

- The problem was for the Japanese,

they had gone so far with their assault,

they could not deliver the code books to all the new sites

because they were too far away.

[dramatic music]

- [Narrator] The speed and success of the Japanese incursion

in the Pacific means the release of the new code book

has been delayed.

And this buys Rochefort more time

to decode yet more of JN-25.

By late spring, 1942,

Rochefort and his team have pieced together

almost all of Yamamoto's battle plan

for the occupation of Midway.

- In his briefing to Nimitz on the 25th of May,

he told Nimitz that Japanese were coming with four carriers.

Now he was pretty sure of that by now.

Probably two battleships as well,

some cruisers, some destroyers.

So that was the strength of the Japanese fleet.

[ominous music]

- [Narrator] The decoded messages reveal

Yamamoto is planning to have significant air support

and naval firepower in place to ambush the U.S. carriers

as soon as they arrive at Midway.

Little does he know that Rochefort has given Nimitz

everything he needs for the Americans

to scupper Yamamoto's ambush plans.

- Nimitz sent almost everything he could get his hands on

that would fly out to Midway's airfield.

His hope was that as the Japanese approached Midway

in order to bomb it, neutralize it for their invasion,

he could then launch an attack on them

where they would least expect it.

[dramatic music]

- [Narrator] And Admiral Nimitz

has another significant advantage over his enemy.

Yamamoto believes he will only be taking on

two U.S. aircraft carriers.

- The Japanese believed they had sunk two American carriers

in the Coral Sea.

That meant the Americans had only two left.

So the fact that the Japanese were sending four carriers

out to take care of them, should be an easy walkover.

- [Narrator] But master strategist and poker player Yamamoto

has for the first time badly overplayed his hand.

- There weren't two American carriers.

There were three,

because the Yorktown did not sink in the Coral Sea.

And Nimitz made sure that its repairs were rushed

to get her back out to sea in time to join the other two.

[solemn music]

- [Narrator] Rochefort's advanced warning

means the U.S. carriers will be at Midway

long before Yamamoto's attack force.

They'll have 360 aircraft already lying in wait

for Japan's 248 planes.

Nimitz is going to ambush Yamamoto's ambush.

- Midway is without a doubt

one of the most significant naval engagements

in the history of modern warfare,

and probably the single most important naval battle

of the Second World War.

- [Narrator] The outcome for the U.S. stands or falls

on whether Rochefort's code cracking is correct.

[propellors whirring]

[suspenseful music] [guns firing]

On June 4th, 1942,

America launches its own surprise attack

on Yamamoto's Japanese fleet.

- The Americans launch early enough

so they catch the Japanese at sixes and sevens.

All these dive bombers arrive over the Japanese carriers

at about the same time.

The Japanese barely have a chance to react.

The American dive bombers scream down on them

at a 70 degree angle and drop their bombs.

[bombs exploding]

[solemn music]

- By midday, three of the four Japanese carriers

are on fire and sinking.

By morning the next day, the fourth one will also have sunk.

- That changes the balance of power in the Pacific.

- Midway is a disaster for the Imperial Navy.

- [Narrator] The U.S. codebreakers,

headed up by Joseph Rochefort,

have been fundamental in the victory against Yamamoto

and the Japanese Imperial Navy at Midway.

- Without the breaking of the JN-25 code,

the outcome could have been very different.

[suspenseful music]

- [Narrator] After Midway,

the war in the Pacific turns increasingly America's way.

[somber music]

Yet, despite suffering such a humiliating defeat,

Japan fails to believe that its super code

has been compromised.

Almost a year after the battle of Midway,

the same JN-25 code is still in use.

But now the complex code that helped the Japanese

plan their full scale invasion

will become the downfall of one of its key architects,

Admiral Yamamoto.

On April 14th, 1943,

U.S. codebreakers decode another vital Japanese message.

- It was the travel itinerary of Admiral Yamamoto,

who was flying down from Rabaul to Bougainville

to talk to some of the Japanese outposts

and boost their morale.

[dramatic music]

Ridding the Imperial Japanese Navy

of their most talented commander

would be a great strategic success.

[propellors whirring]

- [Narrator] Yamamoto is warned against taking the flight

because of the risk of a targeted attack.

But the Admiral is still so convinced JN-25 is uncrackable,

he boards the plane anyway.

- Nimitz authorized a squadron of P-38s

with extended belly tanks to get them to the location,

to fly out to Bougainville and shoot down Admiral Yamamoto,

which they did.

[solemn music]

- [Narrator] Admiral Yamamoto dies instantly

when his plane crashes into the jungles of New Guinea.

The head of the Imperial Navy

is the highest profile victim of the cracking of JN-25.

- The Japanese were totally confident

in the security of JN-25, that it couldn't be broken.

It was a misplaced arrogance to believe that any system

would withstand cryptoanalytic attacks

for as long as it did.

- All of it was a product of the contributions

made by the cryptoanalysts at Station Hypo

led by Joe Rochefort.

[light music]

- [Narrator] Despite his work being directly responsible

for turning the war in the Pacific in America's favor,

Rochefort's maverick attitude and willingness to bypass

his boss in Washington, DC, finally catches up with him.

- Redmond gave Rochefort a sort of take it or leave it order

with the reorganization of the Naval intelligence effort

out in the Pacific.

- [Narrator] Joseph Rochefort is reassigned from the Dungeon

in Pearl Harbor and sent to San Francisco.

His new role has nothing to do with codebreaking.

- He's in charge of a floating dry dock.

He never was put in a position

to be the intelligence analyst that he was ever again.

[dramatic music]

- [Narrator] After the war,

Rochefort is finally honored with the Legion of Merit

for his work cracking the code of JN-25.

He does eventually return to his roots,

heading up the Pacific Strategic Intelligence Group

in Washington DC.

Today he's remembered as the orchestrator

of a phenomenal code breaking operation,

without which the Japanese offensive in the Pacific

may never have been stopped.

[energetic music]

[light music]