US intelligence work out Japan's plans, how well could it go?

Best thing the USN can do?

Move the Battle fleet back to San Diego (or at least thin it down) - the reason it was at Pearl was as a Deterrence - if they knew war was coming it had failed in that task - beef up the US Army Air Corps with standing 'Dawn patrols' etc and prep the airfields for an air attack not a saboteur style attack

Practice mass scrambles and have as many aircraft ready for combat as possible at any given time

What aircraft did the USAAF have on Hawaii? P40 and P36 IIRC?

P-40 C x 12
P-40 B x 87
P-36 A x 39

Oh and 14 P26 Peashooters

What would it take to increase numbers of P40s?

If they had gotten into the air then the KB is having a bad day and this in addition to the AAA being on alert means that their losses are far higher.

However if the IJN got wind that the Battleships had left then they might not bother attacking? Would the attacks in Malaya and PI go ahead?
 

nbcman

Donor
The US Marines also had 10 F4F-3 fighters at Ewa plus an assortment of bombers and other aircraft (38 more). There were two VMFs doing an exercise stateside at the time of PH. Maybe one of those could have also been in PH to increase the Marines fighter strength by another 15-20 F4F-3s. Plus there were 14 F4F-3s on Wake that could have been kept in Ewa as well. These numbers taken from here.
 
....

However if the IJN got wind that the Battleships had left then they might not bother attacking? Would the attacks in Malaya and PI go ahead?

Nagumos orders were to attack the US fleet at sea if located, or attack PH if not. OTL he did have the submarines deployed around the islands, and did have a couple recon planes fly ahead of the air attack. Those reconoitered the harbor, and Lahani Roads - which the fleet occasionally anchored at.
 
Would the U.S. "thin down" the Pac Fleet, knowing war's coming? I doubt it. It depends, to some extent, on how the Japanese attack is uncovered. If it's by traffic analysis of messages out of the Japanese mission in Hawai'i (which would give the game away, had it been done), IMO it's improbable the Fleet would be reduced; it'd be put on higher alert, possibly even augmented: sigint wasn't considered reliable enough to base decisions on, yet.

More to the point, when does the U.S. get the "heads up"? That governs the willingness to keep trying to "frighten" (deter) Japan. If it's a few months early, there might be a willingness to draw down, but I'd bet there'd be a "double down" mentality, instead, not least in the hope of aiding both ROC & Britain.

If, OTOH, Rufe Bratton gets his famous brainwave just a few days or a week later...he looks like a genius, he's a hero, & with any luck goes on to command SIS, or 3d Army, or something fitting. (If, TTL, the Turner brothers end up commanding target barges in the Aleutians, so much the better.)
 
Would the U.S. "thin down" the Pac Fleet, knowing war's coming? I doubt it. It depends, to some extent, on how the Japanese attack is uncovered. If it's by traffic analysis of messages out of the Japanese mission in Hawai'i (which would give the game away, had it been done), IMO it's improbable the Fleet would be reduced; it'd be put on higher alert, possibly even augmented: sigint wasn't considered reliable enough to base decisions on, yet.

More to the point, when does the U.S. get the "heads up"? That governs the willingness to keep trying to "frighten" (deter) Japan. If it's a few months early, there might be a willingness to draw down, but I'd bet there'd be a "double down" mentality, instead, not least in the hope of aiding both ROC & Britain.

If, OTOH, Rufe Bratton gets his famous brainwave just a few days or a week later...he looks like a genius, he's a hero, & with any luck goes on to command SIS, or 3d Army, or something fitting. (If, TTL, the Turner brothers end up commanding target barges in the Aleutians, so much the better.)

Its a 'perfect' response in hindsight - but as you say the US is probably unlikely to move the fleet for a number of reasons - also basing in Pearl allows the fleet to sortie to PI if necessary - so a more realistic response is all ships are kept on a sort of condition 2 alert with guns ready to be manned and steam kept 'up' in several boilers (allowing the ships to fight and if necessary leave the harbour rapidly).

Also if the shore based AAA is fully manned (or at least ready to fight) and more than 3 of the 33 PBYs aloft and searching around the Islands at any given time (all but those 3 were destroyed or damaged beyond repair on the ground) as well as the fighter wings standing too in a more effective manner (i.e. a number of pilots and as many ac armed and ready to fight as possible at any given moment) then 'a day in infamy' is going to be far costlier for the IJN and far less successful in that their strike planes are not going to have free reign (particularly in the first wave) to conduct their attacks.
 
Would the U.S. "thin down" the Pac Fleet, knowing war's coming? I doubt it. It depends, to some extent, on how the Japanese attack is uncovered. If it's by traffic analysis of messages out of the Japanese mission in Hawai'i (which would give the game away, had it been done), IMO it's improbable the Fleet would be reduced; it'd be put on higher alert, possibly even augmented: sigint wasn't considered reliable enough to base decisions on, yet.

More to the point, when does the U.S. get the "heads up"? That governs the willingness to keep trying to "frighten" (deter) Japan. If it's a few months early, there might be a willingness to draw down, but I'd bet there'd be a "double down" mentality, instead, not least in the hope of aiding both ROC & Britain.

If, OTOH, Rufe Bratton gets his famous brainwave just a few days or a week later...he looks like a genius, he's a hero, & with any luck goes on to command SIS, or 3d Army, or something fitting. (If, TTL, the Turner brothers end up commanding target barges in the Aleutians, so much the better.)
Please forgive my ignorance on this - who was Rufe Bratton, and what was his brainwave?
And who were the Turner brothers, and what had they done to deserve a quiet life on the water in the beautiful Aleutians, with pay?
 

nbcman

Donor
Please forgive my ignorance on this - who was Rufe Bratton, and what was his brainwave?
And who were the Turner brothers, and what had they done to deserve a quiet life on the water in the beautiful Aleutians, with pay?

Rufe Bratton = Colonel Rufus Bratton who was one of the analysts who decoded the Japanese 14 part message. It took multiple hours to get word out to PH - and messenger who was delivering the telegram which was sent had to take cover in the ditches as the attack had already begun. If he would have realized the significance of that message quicker or had gotten the wheels of the peacetime US forces moving faster on that Sunday morning, the warning could have been given prior to the attack beginning.

An infamous Turner was Richmond K Turner who commanded the US and Australian forces at the disastrous Battle of Savo Island
 

SsgtC

Banned
Y'know, it occurs to me, if Kimmel has his PBYs up, you might pull that off, the same way Fletcher did at Midway. The difference is, you'd need a whack of good luck to time it just so, & that doesn't seem likely. At Pearl, two waves were planned from the start, not like Midway OTL, where Nagumo dithered & he wasn't expecting USN CVs; at Pearl, he's expecting to catch them, & is going to be ready for them if (when) Halsey's or Brown's birds arrive, since his main targets are ships in harbor.

Now, it's still likely, if Kimmel has an ambush planned, Nagumo doesn't detect USN CVs right away; his recce was absurdly bad. The real problem for Kimmel (& Halsey) is, they won't be expecting an attack from the northwest & are likely about 400nm out of position, so Nagumo will have flown both strikes before a USN response can even launch.

For all the seeming similarity to Midway, it ain't that simple...

Oh, no argument here. It's not simple at all. But if the US knows the IJN is coming, I'm assuming that it's because they've managed to break the Naval Codes, so they would know the fleet is coming from the North. Plus, even if the PBYs miss the fleet completely, having land based fighters up let's them radio a report to the fleet as to which direction they came from and let's them put a trailer up after them to track them back to their fleet. It's complicated, but it would let Halsey hammer the Kido Butai
 
Oh, no argument here. It's not simple at all. But if the US knows the IJN is coming, I'm assuming that it's because they've managed to break the Naval Codes, so they would know the fleet is coming from the North. Plus, even if the PBYs miss the fleet completely, having land based fighters up let's them radio a report to the fleet as to which direction they came from and let's them put a trailer up after them to track them back to their fleet. It's complicated, but it would let Halsey hammer the Kido Butai
It keeps coming back to how much lead time there is. AIUI, Kimmel didn't have enough PBYs for 360deg coverage (meaning the searches are likely to be SW, not NW), so even if there's a warning, there's a good chance Nagumo isn't detected until around the time he launches. That's fortunate for Kimmel (& Halsey). It also likely means the first wave will get a very warm welcome, even compared to OTL's 2d wave, & the 2d wave will fare worse still. It also means USN losses will be a lot lower; ditto AAF a/c losses. (Which raises a question for me: would the local P-40s have been replaced with newer P-40s, or P-38s? If P-40s, or P-36s, survive the attack, there might be a net decrease in fighting power...)

IMO, having Halsey pursue (& damn sure he'd want to) would be about the worst decision Kimmel could make, presuming Nagumo hasn't already given the CVs a pasting despite his own losses.
Please forgive my ignorance on this - who was Rufe Bratton, and what was his brainwave?
Have you seen "Tora! Tora! Tora!"? He's the one who says, "They're going to attack us!" That insight sparked the November "war warning" message...about a week too soon.
And who were the Turner brothers, and what had they done to deserve a quiet life on the water in the beautiful Aleutians, with pay?
Sorry, I keep doing that... (Somehow the wrong name got attached in my memory, & I can't detach it.:teary:) It's him & him, & it's for screwing over Joe Rochefort.

Had FDR been smart, he'd have brought Bratton & Rochefort onto the White House staff... Except, AFAIK, there was nothing like "senior intel advisor" then.
 
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SsgtC

Banned
IMO, having Halsey pursue (& damn sure he'd want to) would be about the worst decision Kimmel could make, presuming Nagumo hasn't already given the CVs a pasting despite his own losses.

Not pursue. One strike. Even if they only figure out where the fleet is based on reports from the fighters over Pearl radioing in what direction the attack came from, it would set the KB up for one beautiful sucker punch. The minute Halsey gets the rough bearing, he launches. Then, one of the land based fighters trails the Japanese back to their fleet giving a much better location for Halsey. The USN hits them hard, concentrating on only the 2 or 3 largest carriers, then gets the hell out of there.
 
Suppose that by August 1941 the US intelligence community reports to Roosevelt and he believes that Japan intends war with USA

By Novemeber US Realizes timing of planned attacks on Pearl Harbor and the Phillipines.

If planning is done right just how bloodier nose can Japan get?

Could this weaken the militarist faction in imperial Japan?
Imperial Japan had at least one spy at Pearl Harbour - Takeo Yoshikawa, according to wikipedia.
Does your point of departure assume that he is picked up and/or turned?
If not, what obvious signs of preparations to receive an attack does he (or other Japanese sympathetic to Imperial Japan in Hawai'i) see and report, and what do the Imperial Japanese Naval headquarters (in the time available) decide and do when they receive said reports?
 
Not pursue. One strike. Even if they only figure out where the fleet is based on reports from the fighters over Pearl radioing in what direction the attack came from, it would set the KB up for one beautiful sucker punch. The minute Halsey gets the rough bearing, he launches. Then, one of the land based fighters trails the Japanese back to their fleet giving a much better location for Halsey. The USN hits them hard, concentrating on only the 2 or 3 largest carriers, then gets the hell out of there.
Unless you (somehow) manage to put Halsey somewhere north or slightly northeast of Oahu & less than 175nm from Nagmo's launch point, it's going to be a pursuit. And he's very unlikely to get that "sucker punch" when Nagumo is expecting a counterattack (which he is). You might (just) make it work by using AAF a/c, with more range, but they'd have to find Nagumo's TF, which IMO is improbable, since they're not trained for that...& any "trailer" is likely to be shot down by IJN escort fighters.

If (& it's a pretty big if) you can have PBYs detect & accurately locate Nagumo (& how you do that when Kimmel's only got a comparative handful of them & is expecting attack from a different direction, I have no clue), you just might get away with AAF counterattck--but now, IMO, you're pushing ASB.

And here's the bigger issue: Nagumo is ready to attack CVs (unlike at Midway, where he got caught switching weapons; that will not happen here). If he detects Halsey at sea, anywhere, the chances are excellent Halsey has his ass handed to him, because Nagumo's fliers are all combat experienced, & none of Halsey's are. Nor are they really ready for how good the A6M is. It's likely, IMO, the U.S. loses Enterprise & Lex (maybe not, if there's reinforcement by Hornet &/or Wasp), tho the aircrew are likely to be able to recover to Oahu, unless contact is made further out. Losses to technicians & supporting personnel aboard the CVs will likely be heavy, too.

I hate to harp on it, but the best outcome is just about the OTL one. It may not look like it, at a glance, but it really was pretty good for the U.S., in all.
Imperial Japan had at least one spy at Pearl Harbour - Takeo Yoshikawa, according to wikipedia.
Does your point of departure assume that he is picked up and/or turned?
If not, what obvious signs of preparations to receive an attack does he (or other Japanese sympathetic to Imperial Japan in Hawai'i) see and report, and what do the Imperial Japanese Naval headquarters (in the time available) decide and do when they receive said reports?
Japan really only had him alone; there were Nisei & Issei, but they were ignorant of what he was doing, not complicit. Truth is, you don't need to pick him up or turn him to uncover the notion Oahu is a target; the amount of message traffic out of the Hawai'i station (consul?) was way out of proportion to what it should've been, & J-19 held clues--between the two, it wouldn't have taken Kreskin to predict an attack.

Would he see more warships in harbor? Yes. More AA guns deployed? Yes. More troops in the streets more of the time? Yes. More fighter patrols? Probably.

Would IJNHQ change their plans to attack? I really doubt it. They expected to achieve surprise, expected things to go perfectly, to the point there was actually poor co-ordination between TBs, level bombers, & DBs, & poor target allocation. (I came across a PDF online criticizing the claim the plan's "brilliance"; good luck finding it, since I can't recall the web address or author...)

One thing about Yoshikawa, tho: if he sees increased preparations by the U.S., does he wonder why? More important, does IJNHQ? Does it lead them to suspect JN-25 is compromised? (Are we presuming it is?) If so, do they change it? Also, if the U.S. has broken JN-25 (not true OTL), does that lead to a broader warning to USG officials not to screw around with Japanese cyphers of any kind, for fear of blowing the gaff? If that happens, there's a reasonable chance the maru code is never copied by an idiotic Customs official:eek: & so is not changed.:cool: (The benefit of this to the Sub Force once the war starts outweighs the harm done by the Mark 6 & Mark 14.)
 
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