UK & US discover Japan's plans in detail October 1941

[Admiral Beez]

We'll have to play with history a bit here, as I'm not sure that even Japan knew what its firm plans for the Pacific war were. But let's have at it.... October 1, 1941 British intel determines that sometime between late November and early December, on the presumption that negotiations with the USA will go no where, Japan will:

• Send the Kido Butai to hit the USN fleet at Pearl Harbour. Japan would need their own intel that the USN was planning to concentrate more ships to PH
• Send the IJA from Formosa and mainland China in an amphibious assault to capture the Philippines, supported by air strikes from Formosa
• Secretly build up a force of IJA, troops ships, AFVs and aircraft in FIC and Formosa to invade Malaya, Burma and DEI (and to force Thailand to accept Japan's terms)

After taking a day to ponder the information, Churchill and his Intel chief informs FDR and his staff. FDR takes a few days to ponder and settle on a POV. October 5, 1941 FDR and Churchill and their respective staff begin plans to counter this. What happens now? Britain is taking losses in North Africa, the North Atlantic and the Germans appears to be unstoppable in Russia. The US still has a small army.

One thing I think would change is Churchill is now convinced that Force Z, planned to sail later that month is no deterrent, and that in fact no deterrent is likely possible. So, preparing for war is likely.

 

[Thumpalumpacus]

With more than a pinch of hindsight, pulling back to the West Coast and playing rope-a-dope would be smart money for the USN at PH, but Japanese intel on-scene would likely pick up on the absence of ships and cause some change in plans.

What the Americans would actually do is probably try to rush more reinforcements to HI and the CenPac islands, resulting in a bigger bag for the Japanese, perhaps.

The Brits would have two options, I think -- keep Zed at Ceylon, or put it in Singapore ready to GTFOOD when the balloon went up but at least they have more options than sitting by and watching Singapore fall from Colombo.

 

[Admiral Beez]

The Brits would have two options, I think -- keep Zed at Ceylon, or put it in Singapore ready to GTFOOD when the balloon went up.

Force Z doesn't sail from the UK until late October, so there's time to reconsider what's being sent. It would be pure negligence not to do otherwise.

Now that Britain knows that the seemingly small IJA force in FIC is just a feint covering a much larger invasion group, London will have to act. What and when do we tell the DEI?

 

[Thumpalumpacus]

Force Z doesn't sail from the UK until late October, so there's time to reconsider what's being sent. It would be pure negligence not to do otherwise.

Now that Britain knows that the seemingly small IJA force in FIC is just a feint covering a much larger invasion group, London will have to act. What and when do we tell the DEI?

I'd tell the future ABDA powers entirely as I learned of it. I'd put forces close to the zone and keep my eyes and ears open. One of the great weaknesses of ABDA is obviously their lack of group cohesion. It might be better to pull them back to Oz and have them spend a few months working up together before the balloon goes up?

I don't know if Darwin could accommodate and supply such a fleet, but if so, perhaps gathering them there and getting them to train together could avoid some of the worse aspects of the bungled battle for Indonesia. As in this case and many others (off Guadalcanal, anyone?) scratch forces have an appalling habit of falling apart.

And yes, this post is loaded with monohindsightium glutamate. Beware the MHG.

 

[EwenS]

RN plans as at Aug 1941 for capital ship and carrier dispositions taking account of ship availability. From Roskill.

Correction about Force Z. It didn't exist until 2nd Dec and the two capital ships making it up did not come together until 30th Nov.

Repulse arrived at Durban, South Africa on 3rd Oct as escort to convoy WS11. She then spent most of Oct & Nov on convoy escort and hunting German raiders with Hermes until arriving in Ceylon on 22nd Nov.

PoW left the U.K. on 24th Oct with 3 destroyers as Force G. It arrived Colombo 28th Nov, linked up with Repulse on the 30th and sailed for Singapore. It only became Force Z on arrival at Singapore on 2 Dec.

In terms of a speedy creation of an Eastern Fleet the best available is to rob the Med Fleet of QE, Valiant & Barham plus whatever cruisers and destroyers can be scraped together. That is what basically happened historically except that by the end of the year Barham had been sunk & QE & Valiant damaged.

But there is a much greater political problem to address. Historically, Brooke-Popham's orders prevented him from taking any offensive action until the Japanese struck until changed at the very last minute. So do the rules of engagement change? Is Operation Matador implemented? Does that just trigger an earlier Japanese response? Does an extra couple of months give enough time to strengthen the defences in northern Malaya?

Operation Matador (1941) - Wikipedia

en.m.wikipedia.org

I very much doubt that Darwin can be used as a fleet base for more than a very short period. For one thing it doesn't have the oil fuel reserves or the defences. Trincomalee has the necessary oil reserves, second only to Singapore.

Also remember that this is the time of the build up to Operation Crusader in the Western Desert. That kicked off on 18 Nov and lasted till the end of the year. Even if you cancel it, it is unlikely that you can shift troops in time to forestall a Japanese invasion on the historical timescale. Maybe some RAF assets can be made available, but they would need flown across India and down to Malaya. And you still have the problem of moving the related ground crews and support equipment (very few transport aircraft around at that time). How long to pack up, arrange troop shipping, move to Malaya, unpack and reorganise?

There are significant assets en route to the Middle East at the time but their U.K. departure was just too late to allow diversion to be useful. Like 18th Inf Div whose departure was already organised on 1 Oct and took place at the end of the month. And two Hurricane wings totalling 7 squadrons.

Time doesn't allow any significant US reinforcement of the Philippines. With adequate warning then perhaps the PH defences can be on full alert and carrier dispositions changed. Maybe a Midway can be triggered in Dec 1941. Maybe Yorktown can be despatched to the Pacific a couple of months earlier giving 3 operational carriers on 7 Dec. Maybe Saratoga's refit can be speeded up to give 4 available.

 

[Admiral Beez]

In terms of a speedy creation of an Eastern Fleet the best available is to rob the Med Fleet of QE, Valiant & Barham plus whatever cruisers and destroyers can be scraped together. That is what basically happened historically except that by the end of the year Barham had been sunk & QE & Valiant damaged.

But there is a much greater political problem to address. Historically, Brooke-Popham's orders prevented him from taking any offensive action until the Japanese struck until changed at the very last minute. So do the rules of engagement change? Is Operation Matador implemented?

I'm not sure sending a three battleships earlier to Singapore will mean much to Japan, unless it's part of a larger/better reinforcement for the army and RAF. With Russia and the convoys on the ropes and Britain seemingly losing every engagement in North Africa I wonder what Churchill will do even if he's convinced Japan will start their offensive soon.

I think Britain needs to make immediate changes diplomatically and with ROE. It's too late to swap out Percival for Monty or someone more capable. I believe that small additions to Malaya's defence could make a big difference. Send in perhaps sixty cruiser or infantry tanks, ideally something armed with HE shells. Return perhaps six RN submarines. And announce to Japan total exclusion zone on the Malay coast and that anything found will be fired upon without warning. And give Thailand the info that Japan is going to invade and ask them to allow British forces in to act alongside the Thai army. What about publicly announcing Japan's plans so that the world knows? That might compromise sigint.

 

[33k in the air]

What the Americans would actually do is probably try to rush more reinforcements to HI and the CenPac islands, resulting in a bigger bag for the Japanese, perhaps.

I'd expect the opposite. The U.S. can lay a trap for the coming Japanese. Get submarines out and patrolling for the incoming IJN fleet; get the fighters on Hawaii on stand-by, ready for immediate action; get more radar stations set up and make sure that information is quickly relayed to commanders for air defense so as to be able to get the fighters up to intercept the incoming Japanese; bring all the USN ships up to combat readiness to make them harder to sink and able to respond with anti-aircraft fire even quicker.

The IJN raid would be walking into a hornet's nest.

 

[Admiral Beez]

I'd expect the opposite. The U.S. can lay a trap for the coming Japanese.

Like Operation Kutuzov, where the Soviets set a trap for the Germans at Kursk. If the RN can find a half dozen subs of their own they can set their own trap in the Gulf of Thailand.

 

[Thumpalumpacus]

I'd expect the opposite. The U.S. can lay a trap for the coming Japanese. Get submarines out and patrolling for the incoming IJN fleet; get the fighters on Hawaii on stand-by, ready for immediate action; get more radar stations set up and make sure that information is quickly relayed to commanders for air defense so as to be able to get the fighters up to intercept the incoming Japanese; bring all the USN ships up to combat readiness to make them harder to sink and able to respond with anti-aircraft fire even quicker.

The IJN raid would be walking into a hornet's nest.

The submarines would go out in any event and you've got a good point about that (never mind the torpedo problems, as that invokes hindsight. I don't know how many spare radar sets the Army or Navy had around to put up to support the Opana Point set and (I think) one or two others in the islands? But possible.

The best thing would be to have Short not lining up his planes to defend against sabotage, but scatter them at readiness.

No doubt the ships should be combat-rigged, but which direction would they head? And by the thinking of the day, they'd probably sortie from harbor, meaning that yes, they might be harder to sink, but in the aftermath impossible to recover.

Knowing the Japanese were coming would probably mitigate the losses, but I don't think it would change too much. If the American ships were caught at sea, that'd be even worse, but that's obviously hindsight.

 

[SaparotRob]

PH could have barrage balloons deployed which would really screw up the torpedo and dive bomber runs. Torpedo nets too. Fighters warmed up and armed Anti aircraft artillery manned and ready. I doubt that all that activity would go unnoticed to prying eyes.

 

[Tkdog]

If the prying eyes did see it… what could they do?

 

[EwenS]

Like Operation Kutuzov, where the Soviets set a trap for the Germans at Kursk. If the RN can find a half dozen subs of their own they can set their own trap in the Gulf of Thailand.

Actually if the "Allies" are co-operating to a much greater extent, then there is no need for the RN to send submarines east. There are plenty USN Asiatic Fleet and Dutch in theatre already (29 & 15 respectively). Historically the problem is twofold:-

US - too many tied up alongside at Cavite / Manila on 8th Dec. Within a couple of days of the outbreak of war the US Asiatic Fleet sailed 18 on war patrols with more following when refit work had been completed or damage sustained on day 1 repaired.

Dutch - Britain had an agreement with the Dutch about the control and use of some of their subs. But it wasn't to come into effect until the outbreak of war. That happened and IIRC 8 Dutch subs went immediately to Singapore and then onto war patrols in the Gulf of Thailand (one was subsequently sunk when it strayed into a British defensive minefield off the Malayan coast).

Now with prior knowledge of an attack all these submarines could have been on patrol in the South China Sea / Gulf of Thailand tracking Japanese convoys and ready to sink them immediately the orders are given.

 

[Admiral Beez]

If in October 1941 the Australians are made aware of Japan's plans they may demand that their troops in North Africa and warships in the MTO and Atlantic come home immediately. And Canada may not be been keen to send their two regiments to Hong Kong.

 

[SaparotRob]

If the prying eyes did see it… what could they do?

Inform their masters that Pearl isn't going to be caught by surprise. Maybe call off the PH strike?

 

[Thumpalumpacus]

If the prying eyes did see it… what could they do?

The Japanese could perhaps shift Kido Butai to the SWP and aid the PI campaign, in the meantime hoping to get Kanto Kessen put into play?

 

[Jagdflieger]

The first question coming to my mind would be, why did Japan want to attack Pearl in the first place?
IMO
1. Their primary objective was to gain full control of China and ensure that the Soviet-Union would not attack them in Manchuria.
If so, the beginning "friendship" between Pay My Check and the USA must have worried them in regards to the USA coming in to help China - rather then helping Britain.
2. They wouldn't be secure in attacking the Philippines with the USN Pacific fleet being intact.
3. Malaya, French-Indochina, Dutch East Indies - they could take on and even be quite safe in assuming that the USA wouldn't bother to much.
Australia probably only comes to their mind in regards to the USA getting onto the warpath with Japan

If so - I think the Japanese would have proceeded with their intentions towards South-East-Asia - hoping that the USA wouldn't get ideas.

But whatever scenario the Japanese would have favored or placed their priority onto, the "danger" USA was always present
Bottom line - Japan must come up with an idea to knock out the USN Pacific fleet.

My second question in this scenario - would the Japanese be aware of the betrayal in regards to those three alternatives/planed operations?

 

[Tkdog]

I have trouble imaging they could have shifted from Pearl to the south easily. Certainly a change in November seems implausible. Would they have had material assets, especially fuel, to make that happen? Did they have the information aboard to brief the pilots on such a mission? They couldn't email them intel reports, charts, etc. would they have what they need?

My point being- a "hey they seem to know you're coming" message from any assets they had on Oahu is likely a go anyway/return to base decision. Especially as heading south would have left the pacific fleet unscathed between them and home.

 

[Jagdflieger]

....I have trouble imaging they could have shifted from Pearl to the south easily....

Why? historically the Japanese did attack Pearl, the Philippines and South-East-Asia more or less simultaneously.
Bringing in their Naval assets (free from engaging Pearl and the Philippines) towards South-East-Asia would have contributed even more to their already Blitzkrieg like actions - or
this part of the IJN and Japans Taiwan based Air-force would have been set aside as a "guarantor" for keeping their Pacific flank protected in case the USA would have decided to disagree with Japans war-plans.

 

[Thumpalumpacus]

I have trouble imaging they could have shifted from Pearl to the south easily. Certainly a change in November seems implausible. Would they have had material assets, especially fuel, to make that happen? Did they have the information aboard to brief the pilots on such a mission? They couldn't email them intel reports, charts, etc. would they have what they

Yeah, it seems to me the fuel is the biggest obstacle/objection to my imagined response, and it is indeed a strong one. I'm not so up on Japanese tanker tonnage but I'm pretty sure it was rather small and thus difficult to redirect. It would depend on how much fuel the alleged 9 (? -- Census Bureau reports this, but it's an odd source so I'd welcome correction -- sounds too large for me) tankers accompanying KdB itself had. But considering that they're probably not doing the high-speed run-in before the attack on PH, the fleet itself should be using a bit less fuel. If there actually were nine tankers of the capacity described below, and if IJN actually had that fuel on hand, perhaps. But then you'll have to gather the BBs for Kessen Kantai.

ETA: the Japanese apparently had 17 Kawasaki-class tankers each with a capacity of about 95,000 bbl, about 15,000 tons, according to my math. They had to support the drive south, the KdB at PH, the fleet attacking PI, and also import oil, so yeah, I'm not sure they'd have the tanker hulls for all that.

No doubt they had other tankers as well, so I'm not sure how useful this envelope calculation is.

 

[Admiral Beez]

I have trouble imaging they could have shifted from Pearl to the south easily.

I agree. Especially as sea transport aide, taking Malaya, DEI and the Philippines is an IJA affair. The IJA and IJN do not get along.

Why? historically the Japanese did attack Pearl, the Philippines and South-East-Asia more or less simultaneously.

I'd describe it as the Navy and Army planned and executed their plans more or less separately. Did the Kido Butai or the IJN's battlefleet ever provide air cover or NGFS for the IJA's offensives?