National Hari Kari - The Japanese attack on Pearl Harbour

[parsifal]

The 70th anniversary of Pearl Harbour and the outbreak of the Pacific War is just about upon us. I thought it appropriate tyo post some kind of tribute and some kind of narrative to describe the events and decisions in the final lead up to the outbreak of war.

The entries I plan to post are based on John Costello's Book "The Pacific War", taken from the "war warnings" section of the book chapter bearing the same name as this thread. Please feel free to post as you see appropriate, however, in the initial term, I would like to limit the discussion from November 7, when Yammamoto signed Operational order No 2 for the attack on Pearl, through to about the end of December or January. The main concentration is December 7th (US, 8th December west of the IDL). I will also use some parts of David Browns "Carrier Operations "and perhaps Norman Polmars "Carrier Warfare" to flesh out bits of the discussion. But please, feel free to post on this subject area as you see appropriate.

This was the period of Japanese offensive, when it seemed they could do no wrong. But we should not forget the heroism and sacrifice made by all combatantsin that time frame .....

 

[davebender]

IMO any realistic discussion of the Japanese decision to go to war with the USA should start on July 26th, 1941. That's when President FDR began a massive military build up in the Philippines. Diplomatic brinksmanship that backfired when Japan chose to fight rather then back down unilaterally.

Perhaps if the USA had agreed to remove heavy bomber groups and long range submarines from Luzon in return for Japan withdrawing from IndoChina they might have brokered a deal. Just as President JFK agreed to remove nuclear missiles from Turkey in return for the Soviets removing nuclear missiles from Cuba. You've got to make a diplomatic concession if you want a diplomatic concession in return.

 

[tyrodtom]

31 P-40's sent in Aug. 41, 50 more in Oct., 9 B-17's in Sept, and 26 more in Nov. They had planned to have 165 B-17's by Mar. 42. I don't think those amount to a " massive " military build up in anyones mind.
The early B-17s when bombed up didn't have the range to threaten anyone ourside of the Phiippines, they could not reach China or Japan and return. The only thing the 165 B-17's were a threat to was if Japan waited till after March 42 to invade the Philippines.

 

[parsifal]

Nov 5 1941
24 hours after the Japanese Supreme War Council had decided to that in a months time it would be impossible to launch military operations becuae of the winter weather , prime minister Tojo persuades the Imperial Council into postponing the deadline until December 1 to allow for a final attempt "to solve the problem by diplomacy". To assists the struggling Nomura in this final bid for peace, he sends Saburo Kurusu to Washington as the chief negotiator in the final round of talks.

Kurusu's detailed knowledge of the US and his American wife were expected to be valuable assets in the negotions. Tokyos last plan for peace incorporated two parts. The first....proposal A called for a complete settlement of the China incident with a limited withdrawal of Japanese Troops. The second proposal, Proposal B was a fall back to buy time with a modus vivendi in which japan would halt further military ops in return for the US supplying a million gallons of Aviation fuel.


The Americans knew of this plan even before it had been submitted, through its MAGIC Intercepts. The president instructed Hull to "strain every nerve" and "do nothing to precipitate a crisis". Fully aware that negotiations now hung by a slender thread, roosevelts aim was to spin the negotiations out for as long as possible. On November 7 he sent a message to Churchillexoplaining that he was moving very cautiously towards meeting Chiang Kai Sheks laterst military demands. he told Churchill that he was counting on time for "continuing efforts to strengthen our defences in the PI, paralleled by similar efforts by you in Malaya, which will increase Japans hesitation towards war."

Neither the President nor Churchill wanted a showdown that would force Japan into war, which would be likley to interefere in the unofficial partnership they had formed in the struggle against Hitler. Yet Churchill for one knew that by urging Roosevelt to adopt a no compromise stance this was more likley to prompt Japan to attack in the Far East against the Dutch and the British. Even though no formal agreement existed at that time, Churchill was confident that the AQmericans would have no alternative to interveneIndeed, this became more and more evident as the collission between Japan and the West became moree and more obvious as that las t month of peace passed.

Impact was made inevitable by the US to make even the smallest concessions to the Japanese. This refusal to compromise stemmed from two sources, the experiences of Munich in 1938 had completely hardened the Allies and the US to any solutions that involved compromise, and a mistaken belief in the power of the military presence in the far east. the Americans in particular placed far too much emphasis on the PI and Singapore deterrents, and gave too little attention to the exposed fleet position at Pearl.

 

[parsifal]

November 7

The pearl harbour Strike Force, after a dress rehearsal involving all six of the participapting carriers, have orders signed by Yammamoto. Known as "Operational Order Number 2" it sets December 8 as Y Day for the attack on Oahu (7 December Hawaii time)


Contain within the general Operations Orders were the specific orders, which were not released until November 23. They were rather confusingly referred to as "operational order no 3"

The following is an English translation of those orders:
(Part I)
"23 November 1941
To: Carrier Striking Task Force
1. The Carrier Striking Task Force will proceed to the Hawaiian Area with utmost secrecy and, at the outbreak of the war, will launch a resolute surprise attack on and deal a fatal blow to the enemy fleet in the Hawaiian Area. The initial air attack is scheduled at 0330 hours, X Day. Upon completion of the air attacks, the Task Force will immediately withdraw and return to Japan and, after taking on new supplies, take its position for Second Period Operations. In the event that, during this operation, an enemy fleet attempts to intercept our force or a powerful enemy force is encountered and there is danger of attack, the Task Force will launch a counterattack.
2. The disposition of Force will be as shown on Chart 1.
3. The Operation of Each Force.
a. General
While exercising strict antiaircraft and antisubmarine measures and making every effort to conceal its position and movements, the entire force (except the Midway Bombardment Unit) in accordance with special orders will depart as a group from Hitokappu Bay at a speed of 12-14 knots. The force refueling en route whenever possible will arrive at the standby point (42 N, 165 W). In the event bad weather prevents refueling en route to the standby point, the screening unit will be
[6]. General outline.
MAP: Planned Track of Carrier Striking Force For Pearl Harbor Attack. (not shown in this article)[Page 9]

CHART: Chart 1, Disposition of Forces.
Task Force
Classification: Air Attack Force
Commander: 1st Air Fleet Commander
Strength: 1st Air Fleet
1st Carrier Division
CV Akagi
CV Kaga
2nd Carrier Division
CV Hiryu
CV Soryu
3rd Carrier Division
CV Zuikaku
CV Shokaku
Duties: Air Attacks

Classification: Screening Unit
Commander: 1st Destroyer Squadron Commander
Strength: 17th Destroyer Division
(Nagara Flagship)
18th Destroyer Division
(Akigumo Flagship)
Duties: Screening and escort
Classification: Support Force
Commander: 3rd Battleship Division Commander
Strength: 3rd Battleship Division
(less the 2nd section)
8th Cruiser Division
Duties: Screen and support

Classification: Patrol Unit
Commander: 2nd Submarine Division Commander
Strength: I-19 (Flagship)
I-21
I-23
Duties: Patrol
Classification: Midway Bombardment Unit

Commander: 7th Destroyer Division Commander

Strength: 7th Destroyer Division
(less the 2nd section)

Duties: Midway air base attack

Classification: Supply Force
Overall Commander: Kyokuto Maru Commander
Classification: Supply Force (1st Supply Unit)
Commander: Kyokuto Maru Commander
Strength: Kenyo Maru
Kyokuto Maru
Kokuyo Maru
Shinkoku Maru
Akebono Maru
Duties: Supply

Classification: Supply Force (2nd Supply Unit)
Commander: Toho Maru Inspector
Strength: Toho Maru
Toei Maru
Nippon Maru
Duties: Supply

ordered to return to the home base. Subsequent to the issuance of the order designating X Day (the day of the outbreak of hostilities), the force will proceed to the approaching point (32 N, 157 W).

Around 0700 hours, X-1 Day the Task Force will turn southward at high speed (approximately 24 knots) from the vicinity of the approaching point. It will arrive at the take-off point (200 nautical miles north of the enemy fleet anchorage) at 0100 hours X Day (0530 Honolulu time) and commit the entire air strength to attack the enemy fleet and important airfields on Oahu.

Upon completion of the air attacks, the Task Force will assemble the aircraft, skirt 800 nautical miles north of Midway, return about X + 15 Day to the western part of the Inland Sea via the assembly point (30 N, 165 E) and prepare for Second Period Operations. In the event of a fuel shortage the Task Force will proceed to Truk via the assembly point.

The force may skirt near Midway in the event that consideration of an enemy counter-attack is unnecessary due to successful air attacks or if such action is necessitated by fuel shortage.

In this event, the 5th Carrier Division with the support of the Kirishima from the 3rd Battleship Division will leave the Task Force on the night of X Day or the early morning of X + 1 Day and carry out air attacks on Midway in the early morning of X + 2 Day.

If a powerful enemy force intercepts our return route, the Task Force will break through the Hawaiian Islands area southward and proceed to the Marshall Islands.

b. Patrol unit
The patrol unit will accompany the main force. In the event the screening unit is returned to the home base, the patrol unit will screen the advance of the main force and the launching and the landing of aircraft. After the air attacks, the patrol unit will station itself between the flank of the main force and the enemy. In the event of an enemy fleet sortie, the patrol unit will shadow the enemy and in a favorable situation attack him.

c. The Midway Bombardment Unit
The Midway Bombardment Unit will depart from Tokyo Bay around X-6 Day and, after refueling, secretly approach Midway. It will arrive on the night of X Day and shell the air base. The unit will then withdraw and, after refueling, return to the western part of the Inland Sea. The oiler Shiriya will accompany the bombardment unit on this mission and will be responsible for the refueling operation.

d. Supply Force
The supply force will accompany the main force to the approaching point, carrying out refuelings, separate from the main force, skirt 800 nautical miles north of Midway, return to the assembly point by 0800 hours, X +6 Day, and stand by.

4. The Task Force may suspend operations en route to the Hawaiian area and return to Hitokappu Bay, Hokkaido or Mutsu Bay, depending upon the situation.

Commander
Carrier Striking Task Force
Nagumo, Chuichi

 

[parsifal]

Operational Order No 3 (Part II)

"The Hawaiian operations air attack plan has been decided as follows:

1. The Operation of the Air Attack Forces

The force will be 700 nautical miles due north of point Z (set at the western extremity of the Island of Lanai) at 0600 hours X-1 Day and advance on a course of 180 degrees from 0700 hours X-1 Day at an increased speed of 24 knots.

Air attacks will be carried out by launching the first attack units 230 nautical miles due north of Z point at 0130 hours X Day, and the second attack unit at 200 nautical miles due north of Z point at 0245 hours.

After the launching of the second attack units is completed, the task force will withdraw northward at a speed of about 24 knots. The first attack units are scheduled to return between 0530 and 0600 hours and the second attack units are scheduled to return between 0645 and 0715 hours.

Immediately after the return of the first and second attack units, preparations for the next attack will be completed. At this time, carrier attack planes capable of carrying torpedoes will be armed with such as long as the supply lasts.


[7]. General outline.

If the destruction of enemy land-based air strength progresses favorably, repeated attacks will be made immediately and thus decisive results will be achieved.

In the event that a powerful enemy surface fleet appears, it will be attacked.

2. Organization of the Air Attack Units
not included in translation)

3. Targets
a. The First Attack Units
The targets for the first group will be limited to about four battleships and four aircraft carriers; the order of targets will be battleships and then aircraft carriers.

The second group will attack the enemy land-based air strength according to the following assignment:
The 15 Attack Unit: Hangars and aircraft on Ford Island
The 16 Attack Unit: Hangars and aircraft on Wheeler Field
The targets of Fighter Combat Units will be enemy aircraft in the air and on the ground.

b. The Second Attack Units
The first group will attack the enemy air bases according to the following assignment:
The 5 Attack Unit: Aircraft and hangars on Kaneohe, Ford Island and Barbers Point.
The 6 Attack Unit: Hangars and aircraft on Hickam Field.
The targets for the second group will be limited to four

Chart 2
Organization of the Air Attack Units
1st Attack Units CO Commander Fuchida
1st Group CO do
1st Attack Unit CO do
15 Kates each fitted with a 800-kg Armor Piercing Bomb for
level (high altitude) bombing.
2nd Attack Unit CO Lt Cmdr Hashiguchi
15 Kates-Same bombs as 1st Attack Unit.
3rd Attack Unit CO Lt Abe
10 Kates-Same bombs as 1st Attack Unit.
4th Attack Unit CO Lt Cmdr Kusmi
10 Kates-Same bombs as 1st Attack Unit.
1st Torpedo Attack Unit CO Lt Cmdr Murata
12 Kates each fitted with an Aerial Torpedo, Mark 91.
2nd Torpedo Attack Unit CO Lt Kitajima
12 Kates-Same torpedoes as 1st Torpedo Attack Unit.
3rd Torpedo Attack Unit CO Lt Nagai
8 Kates-Same torpedoes as 1st Torpedo Attack Unit.
4th Torpedo Attack Unit CO Lt Matsumura
8 Kates-Same torpedoes as 1st Torpedo Attack Unit.
2nd Group CO Lt Cmdr Takahashi
15th Attack Unit CO do
27 Vals each fitted with a 250-kg Anti-ground (general purpose)
bomb for dive bombing.
16th Attack Unit CO Lt Sakamoto
27 Vals-Same bomb as 15th Attack Unit.
3rd Group CO Lt Cmdr Itaya
1st Fighter Combat Unit CO do
9 Zekes for air control and strafing
2nd Fighter Combat Unit CO Lt Shiga
9 Zekes-Same Mission
3rd Fighter Combat Unit CO Lt Suganami
9 Zekes-Same Mission
4th Fighter Combat Unit CO Lt Okajima
6 Zekes-Same Mission
5th Fighter Combat Unit CO Lt Sato
6 Zekes-Same Mission
6th Fighter Combat Unit CO Lt Kaneko
6 Zekes-Same Mission

2nd Attack Units
1st Group CO Lt Cmdr Shimazaki
6th Attack Unit CO do
27 Kates each fitted with a 250-kg Anti-ground (general purpose
bomb and six 60-kg Ordinary bombs for level (high altitude)
bombing.
5th Attack Unit CO Lt Ichihara
27 Kates-Same bombs as 6th Attack Unit
Chart 2 (Cont'd)
2nd Group CO Lt Cmdr Egusa
13th Attack Unit CO do
18 Vals each fitted with a 250-kg Ordinary bomb for dive bombing.
14th Attack Unit CO Lt Kobayashi
18 Vals-Same bombs as 13th Attack Unit
11th Attack Unit CO Lt Chihaya
18 Vals-Same bombs as 13th Attack Unit
12th Attack Unit CO Lt Makino
18 Vals-Same bombs as 13th Attack Unit
3rd Group CO Lt Shindo
1st Fighter Combat Unit CO do
9 Zekes for air control and strafing
2nd Fighter Combat Unit CO Lt Nikaido
9 Zekes-Same Mission
3rd Fighter Combat Unit CO Lt Iida
9 Zekes-Same Mission
4th Fighter Combat Unit CO Lt Kumano
9 Zekes-Same Mission
or five enemy aircraft carriers. If the number of targets is insufficient, they will select targets in the order of cruisers and battleships.

The Fighter Combat Units will attack the enemy aircraft in the air and on the ground."

 

[parsifal]

operational order Part III

4. Attack Procedure
a. The First Attack Units
(1) With the element of surprise as the principle, attacks will be carried out by the torpedo unit and bomber unit of the First Group, and then the Second Group.

(2) During the initial phase of the attack, the Fighter Combat Units will, in one formation, storm the enemy skies about the same time as the First Group, and contact and destroy chiefly the enemy interceptor planes.

In the event that no enemy aircraft are encountered in the air, the units will immediately shift to the strafing of parked aircraft as follows:

1st and 2nd Fighter Combat Units: Ford Island and Hickam Field.
3rd and 4th Fighter Combat Units: Wheeler Field and Barbers Point.
5th and 6th Fighter Combat Units: Kaneohe

(3) In the event that the advantage of surprise attack cannot be expected due to strict enemy security, the
approach and attack will be made in the order of the Fighter Combat Units, Dive Bombing Units, Horizontal Bombing Units and the Torpedo Attacking Units.

b. The Second Attack Units
All units will storm the enemy skies almost simultaneously and launch the attacks.
Although the general outline of the operations of the Fighter Combat Units corresponds to that of the First Attack Units, the strafing will be carried out according to the following in case there are no enemy aircraft in the air.

1st and 2nd Fighter Combat Units: Ford Island and Hickam Field
3rd and 4th Fighter Combat Units: Wheeler Field and Kaneohe

c. The general outline of attack in the event that enemy aircraft carriers and the main body of the U.S. Fleet are in anchorages outside Pearl Harbor are:

(1) The organization and targets are the same as mentioned above. The First Attack Units of the First Group, however, will increase the number of torpedo bombers as much as possible.

(2) Escorted by the Fighter Combat Units, the Air Attack Units will proceed in a group and attack the designated targets in the order of the enemy fleet anchorages and the Island of Oahu. If attacks on the enemy fleet anchorages progress favorably, however, the Fighter Combat Units and the 2nd Group of the First Air Attack Unit will immediately proceed to the Island of Oahu. Upon completion of the attacks, the anchorage attack unit will return directly to the carriers.

d. Rendezvous for Return to Carriers
(1) The rendezvous point will be 20 nautical miles at 340 degrees from the western extremity (Kaena Point) of the Island of Oahu. The rendezvous altitude will be 1,000 meters. (If this vicinity is covered with clouds, it will be below the cloud ceiling.)

(2) The Attack Units will wait at the rendezvous point for about 30 minutes and return to their carriers, after being joined by the Fighter Combat Units.

(3) While returning to carriers, the Fighter Combat Unit will become the rear guards for the whole unit and intercept any enemy pursuit.

5. Reconnaissance
a. Pre-operation Reconnaissance

Pre-operation reconnaissance will not be carried out unless otherwise ordered.

b. Immediate Pre-attack Reconnaissance
Two reconnaissance seaplanes of the 8th Cruiser Division will take off at 0030 hours, X Day, secretly reconnoiter Pearl Harbor and Lahaina Anchorage and report the presence of the enemy fleet (chiefly carriers and the main body of the fleet).

c. Scouting Patrol
The reconnaissance seaplanes of the 8th Cruiser Division will take off at 0300 hours and will carry out an extensive search of the waters between the enemy and the friendly forces and the waters adjacent to the two channels situated to the east and west of the Island of Oahu. They will observe and report the presence and activities of the enemy sortie force and enemy aircraft on counter-attack missions.

d. Before returning to its carrier, after the attack, an element of fighters designated by the Fighter Combat Unit Commander will fly as low and as fast as circumstances permit and observe and determine the extent of damage inflicted upon the enemy aircraft and ships.

Air Security Disposition No. 1 Method B will be followed from one hour before sunrise until 45 minutes after sunset on the day of the air attack."

 

[mikewint]

Parsifal, absolutely top rate excellent detail sir, as always

 

[parsifal]

Thanks for the compliment, but there are other members with a far greater knowledge that me on particular aspects of this war. I like to think i have a good basic knowledge of the overall events, but there are guys here that know what colour underwear a particular flyer was wearing for a particular combat.

The pearl harbour attack was, in my opinion, the most thoroughly planned operation of the war. As a military exercise it was a brilliant piece of soldiering. as a piece of national policy it was a near disaster, a death sentence. Japans moments of glory were spectacular, and spectacularly brief. they were the mouse that roared...they scared the elephant for a brief moment, and then were crushed under the weight of its muscle and courage

 

[davebender]

4 American heavy bomber groups were enroute to the Philippines as of December 1941. If your numbers are correct that works out to about 40 B-17s per bomber group.

Submarines, U.S. Asiatic Fleet, 8.12.1941
The USN had 29 modern long range submarines based on Luzon as of December 1941. Plus 3 submarine tenders and a submarine rescue ship. I have don't know if this was the planned total or if more were enroute as of the Japanese attack.

Hundreds of fighter aircraft and other reinforcements were also enroute to the Philippines. However I think it was the long range submarines and bombers which Japan considered a threat. From Luzon they had the potential to strike Japanese shipping in the China Sea.

1941 Japan didn't have the benefit of hindsight. They had to assume USN torpedoes would work and U.S. Army Air Corps bombers were capable of hitting ships at sea.

 

[parsifal]

Its fair to say that the US attached a lot of importance on the deterrent value of their far eastern forces. It was a serious miscalculation. Japans primary concern, was not the miltary deterrents that had been sent to the theatre, it was the resources embargo that had been imposed on her

 

[davebender]

Do you have evidence to support that statement?

 

[parsifal]

November 7

Washington: Ambassador nomura arrives at the White House to present Proposal "A". He is seeking a comprehensive settlement. Knowing that the "modus Vivendi" would be Japans next move, Secretary hull stalls. The President rejects an immediate reply by telling Nomura "Nations must think 100 years ahead".

Proposal "A" is formally rejected on November 14. The US insists on the total evacuation of all Japanese troops from China (I am unsure if that includes Manchuria, though I think it does). The Americans make this demand, knowing it would be unnacceptable to the Japanese, they had fought a four year long war, in which casualties exceeded 1 million men. It was simply too high a price for the Japanese to pay for peace.

The next day (November 15, Bishop walsh's effort to mediate was dismissed by the State Department as naive. Hull also concludes after a meeting with Kurusu, that he (kurusu) is "deceitful". MAGIC intercepts pick up instructions to Japanese Consul Kita, in Honolulu, to submit his "ships report" on a twice weekly basis. (this vital clue was not passed to Pearl Harbour Command)

 

[parsifal]

Do you have evidence to support that statement?

im re-reading Costellos 740 book that deals with this matter in great detail. The Japanese were unconcerned by the military buildup as it existed in December 1941. Maybe it woulod have changed after further reinforcement, but not in the lead up to the war.

 

[buffnut453]

The pearl harbour attack was, in my opinion, the most thoroughly planned operation of the war. As a military exercise it was a brilliant piece of soldiering.

The Pearl Harbor attack was thoroughly planned and supremely executed all except for the target selections. Leaving the huge oil tanks untouched was a mistake of monumental proportions. The best way to prevent the USN regaining the offensive initiative was to prevent ships from sailing and the best way to do that was to destroy existing oil stocks at Pearl and then interdict, using submarines, any attempts at resupply from the US west coast. Such a strategy would surely have limited the US Pacific Fleet's ability to operate and might have given Japan the 12 months of tactical and operational freedom they had originally sought from the Pearl Harbor attack.

Aside from this, the other key unanswerable question is whether the US would have gone to war had Japan attacked Malaya, Thailand and the Dutch East Indies? Although the US military presence in the Philippines presented a threat to the Japanese flank, that threat was only real if the US decided to go to war with Japan. Even in late 1941, I suspect that continuing isolationist sentiment made it unlikely that Roosevelt would have taken his country to war to protect British or Dutch possessions in the Far East. Japanese leaders were so immersed in a national military inferiority complex, and with perceptions that "everyone was against them" that they failed to notice the lack of any tangible military cooperation between Britain and America in the Far East. This must count as one of the worst political mis-readings of a situation.

Just a few thoughts to keep the thread interesting

 

[parsifal]

re the chances of the US coming to the aid of the British and Dutch. Ther was no formal agreement, but Churchill was very confident that the US would come to his aid in that eventuality. I'm pretty sure Costello deal with this issue some more eslewhere in his book (but its 20 years since I read it). I cant help but believe that had been given such assurances by Roosevelt. Feelings were strong in the US against Japanese aggression across the whole of the Far east. It wasnt the uphill battle for roosevelt to garner support for war in the same way that he had found with Germany.

Thus far ther isnt a right or wrong answer on this, its a matter of opinion. People will have to mostly come to their own conclusions about what might have happened......

 

[buffnut453]

I did say it was an unanswerable question...but I think Roosevelt would still have had an uphill struggle to convince Congress to go to war if America had not been attacked. Others may differ. That's what makes the discussion fun.

I do stand by my comments about the targeting idiocy of the Pearl Harbor attack, though.

Cheers,
Mark

 

[parsifal]

I did say it was an unanswerable question...but I think Roosevelt would still have had an uphill struggle to convince Congress to go to war if America had not been attacked. Others may differ. That's what makes the discussion fun.

I do stand by my comments about the targeting idiocy of the Pearl Harbor attack, though.

Cheers,
Mark

You can mostly blame the timidity of Nagumo....chance of a lifetime and he decides to call it a day. Genda reportedly was reduced to tears trying to persuade him for a third strike

Still, the damage caused to the fleet was substantial, and threw Us warplans into chaos. i agree completely that destruction of the fuel farm would have been even better

 

[parsifal]

November 16

Concealed by strict radio silence, the carriers of the IJN sailed independantly from the Inland sea to avert suspicion. Their destination was the remote Tankan Bay in the Kurile Is. To camourflage their movements, Yammamoto orders their call signs transferred to destroyers.


Japanese cypher codes

This is an extremely complex, and even today, incomplete side of the story. There is no doubt that breaking of Japanese coses provided significant advantages to the Allies, but in the lead up to war, Allied abilities, particulalry in military ciphers were limited. perhaps 10% of Japanese military traffic could be read, and the US in particular lacked the assessment and evaluation infrastructure to take advantage of what intell it was receiving

JN-25 was the name given by codebreakers to the chief, and most secure, command and control communications scheme used by the Imperial Japanese Navy (IJN) during and slightly before World War II (it was the 25th Japanese Navy system identified). Introduced in 1939 to replace Blue Code (which dated back to WWI) , it was an enciphered code, producing five numeral groups in the traffic which was actually broadcast. It was frequently revised during its lifetime, and each new version required a more or less fresh cryptanalytic start. New code books were introduced from time to time and new superenciphering books were also introduced, sometimes simultaneously. In particular, JN-25 was significantly changed on 1 December 1940, and again on 4 December 1941, just before the attack on Pearl Harbor. That and the inability of the US codebreakers to even decode callsigns and ship deployments blinded the US sigint effort at a critical moment. It was the Deecember 04/41 edition of the JN-25 system which was sufficiently broken by late May 1942 to provide the forewarning which led to the U.S. victory at the Battle of Midway.

British, Australians, Dutch and Americans cooperated on attacks against JN-25 beginning well before the Pearl Harbor attack. However, the Japanese Navy was not engaged in significant battle operations until late 1941, so there was little traffic available with which to work. Before then, IJN discussions and orders could generally travel by more secure routes than encrypted broadcast, such as courier or direct delivery by an IJN vessel. Publicly available accounts differ, but the most credible agree that the JN-25 version in use before December 1941 was not more than perhaps 10% broken at the time of the attack, and that primarily in stripping away its superencipherment. JN-25 traffic increased immensely with the outbreak of naval warfare at the end of 1941 and provided the cryptographic "depth" needed to succeed in substantially breaking the existing and subsequent versions of JN-25.

The American effort was directed from Washington, D.C. by the U.S. Navy's signals intelligence command, called OP-20-G. It was centered at Pearl Harbor at the U.S. Navy's Combat Intelligence Unit (Station HYPO, also known as COM 14), commanded by Commander Joseph Rochefort. With the assistance of Station CAST (also known as COM 16, jointly commanded by Lts Rudolph Fabian and John Lietwiler) in the Philippines, and the British Far East Combined Bureau in Singapore, and using an IBM punched card tabulating machine (when available), a successful attack was mounted against the JN-25 edition which came into effect on 1 December 1941. Together they made considerable progress by early 1942. Cribs were used by exploiting common formalities in Japanese messages, such as "I have the honor to inform your excellency", the known call signs of various ships and the use of formal, stylized titles which were often in known plaintext format.

The Purple cipher (also sometimes referred to as AN-1, used by the Japanese Foreign Office as its most secure system, had no cryptographic connection with any version of JN-25, or indeed with any of the encryption systems used by the Japanese military before or during the War. Purple traffic was diplomatic, not military, and in the period before the Pearl Harbor attack, the Japanese military, which controlled Japanese policy, did not trust the Foreign Office enough to tell it much. JN-25 traffic, on the other hand, was limited to military matters, mostly IJN operational ones, from which strategic or tactical information could sometimes be inferred. Nevertheless, decrypted Purple traffic was very valuable, especially later in the war,

JN-39 was a naval code used by merchant ships (commonly known as the "maru code"), broken in May 1940. and again 28 May 1941, when the whale factory Nisshin Maru II visited San Francisco, U.S. Customs Service Agent George Muller and Commander R. P. McCullough of the U.S. Navy's 12th Naval District (responsible for the area) boarded her and seized her codebooks, without informing Office of Naval Intelligence (ONI). Copies were made, in a clumsy way, and the originals returned. The Japanese quickly realized JN-39 was compromised, and replaced it with JN-40.

JN-40 JN-25 was replaced by JN-40 (just after Midway), which was originally believed to be a code super-enciphered with a numerical additive in the same way as JN-25. However, in September 1942, an error by the Japanese gave clues to the codebreakers at the FECB, Kilindini. Nevertheless, much of the guadacanal and new Guinea campaigns were fought "blind' by the allies as a result of this new code. JN-40 was a fractionating transposition cipher based on a substitution table of 100 groups of two figures each followed by a columnar transposition. By November 1942, they were able to read all previous traffic and break each message as they received it. Enemy Naval warships and military transport was thus trackable, enabling Allied submarines to successfully attack it.

JN-152A simple transposition and substitution cipher used for broadcasting navigation warnings. In 1942 the FECB at Kilindini broke JN-152 and the previously inpenetratable JN-167, another merchant shipping cypher.

JN-167A merchant-shipping cipher (see JN-152).

In 1942 there was an incident involving the Chicago Tribune that nearly compromised the Allied SigInt efforts. In June 1942 the Chicago Tribune, run by isolationist Col. Robert L McCormick, published an article that implied that the United States had broken the Japanese codes. This was a serious breach of national security. The government at first wanted to prosecute the Tribune under the Espionage Act of 1917. For various reasons, including the desire not to bring more attention to the article, the charges were dropped. Evidently, the Japanese dont read the chicago tribune

 

[buffnut453]

I heartily recommend "And I Was There" by Layton for an inside view of US code-breaking efforts, the in-fighting between DC and Pearl, and the issue of what intelligence was provided to Kimmel prior to the attack. It's a fascinating insight.

To cut a long story short, DC deliberately (or through incompetence) withheld vital "political" intelligence from Pearl Harbor, including the state of negotiations, the infamous 14-part message that Kurusu was to deliver to Hull at a given time, and "bomb plot" messages that originated from the Japanese consulate in Hawaii to Japan providing details of which USN vessels where were within the harbour. DC sought to control all "radio intelligence" and consistently fought the assessments coming out of Pearl Harbor both before and after the attack. Unfortunately, Pearl got it right more than DC which only exacerbated the problem. It was the team at PH that identified Midway as a target for attack but DC subsequently took all the credit and replaced Rochefort as the PH team lead. Overall, a very messy situation.