Question about Operation Downfall and US codebreaking

Given that Japanese military communications remained open to US code breakers right up to the end of WWII, did the US top brass realize how well prepared the IJA was for an invasion of Kyushu? Conservative estimates for American casualties topped half a million, so if American leadership also has a detailed breakdown of KETSU-GO (the Japanese plan for the defense of Kyushu) how likely is DOWNFALL to be scrapped? Would the alternative be to simply starve the Japanese into submission, or would atom bombs have been dropped on Japan as fast as they could be manufactured?
 
The raw numbers of men and weapons for the Japanese defense look large. The US Army took those into account. They also looked at the number of trained and equipped infantry, artillery, tank, and support units or formations there were. In terms of trained soldiers the 'armies' defending were actually corps & with not that many actual 'divisions'. On Okinawa the Japanese mustered a huge number of armed men, but a significant number were very recently mobilized reservists, with little training and inadequate weapons. Those and a group of 'militia' regiments or battalions not only proved of poor ability in combat, but the surviving Japanese Army officers found them deserting as the campaign progressed. The Japanese 10th Army operations officer Col Yahara described the Army rear services soldiers as being ineffective when used as replacements in infantry battalions, and when ordered enmass into the battle line in the closing days they tried to avoid the order.

...Conservative estimates for American casualties topped half a million, ...

Thats more of a median estimate. MacArthurs estimate was officially 250,000, or less. Since he avoided counting losses from disease and accident in his other campaigns this number may have been only for combat losses. Macs senior medical officer and staff estimated close to half a million, tho that I am certain included non combat losses. Others estimated higher. Kruger endorsed a estimate from his medical officer that projected much higher than Macs. The US Navy included its expected losses as well as a more pessimistic view of the Army losses, so that number was higher.

The projected or estimated losses reflected internal military politics. Mac was anxious to get on with a invasion, so he argued it would be easy. The Admirals saw no reason for a land campaign and were arguing for a naval blockade. So, they went high on their projections. I'd recommend Skates 'Operation Downfall' as a quick and useful primer for this subject.

Given that Japanese military communications remained open to US code breakers right up to the end of WWII, ...

Most Japanese codes were broken & their messages were easily read in 1945. HOWEVER, there was a notable exception. A code or encryption used by the Japanese Army at division and up levels was not broken. Some other second and third tier codes related to the Army were broken, but the US could not read the Japanese Army radio traffic the same way it could read the Navy commanders, or German field armies ENIGMA encrypted radio traffic.
 
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Given that Japanese military communications remained open to US code breakers right up to the end of WWII, did the US top brass realize how well prepared the IJA was for an invasion of Kyushu? Conservative estimates for American casualties topped half a million, so if American leadership also has a detailed breakdown of KETSU-GO (the Japanese plan for the defense of Kyushu) how likely is DOWNFALL to be scrapped? Would the alternative be to simply starve the Japanese into submission, or would atom bombs have been dropped on Japan as fast as they could be manufactured?

That was actually something that was starting to come to a head in August 1945. Magic and Ultra intercepts, confirmed by photo reconnaissance, revealed the sheer scale of Japanese defensive preparations on Kyushu during the course of July and despite MacArthur's best attempts to suppress it, that intelligence had managed to make it's way to the Navy. As early as May of 1945 Nimitz was expressing the view that Operation Olympic (the planned invasion of Kyushu) was unwise, having advised King on May 25 that he no longer supported Olympic. As the intelligence continued to gather showing massive Japanese forces building in Kyushu many other officers echoed these doubts. When King received a report by MacArthur on 9 August claiming that the intelligence was grossly exaggerated (it was not) and that Olympic was still viable, he immediately forwarded to Nimitz asking for him to append his own comments and then forward the whole thing back to MacArthur. Since King already knew that Nimitz opposed the invasion, Frank is certainly correct in judging that King's intent was to "bring on an explosive interservice confrontation over Olympic." [Richard Frank, Downfall p.276]

Similarly the US Army Air Force was also poised to come out against the invasion. On 11 August, following the receipt of a comprehensive study on the effects of bombing in Germany, Spaatz had issued a new targeting directive turning the bombers away from firebombing of population centres and instead aiming them at Japan's transportation network. That same day Spaatz also wrote in his diary that he intended to write the Assistant Secretary of War, arguing that the invasion should be cancelled so the atomic bombs and the newly retargeted bombers could be allowed to do the job.[p.307]

The Joint Chiefs also were not solidly committed to an invasion. On the 9 August when the first bomb was dropped on Hiroshima, the Joint Plans Committee of the Joint Chiefs forwarded a report titled "Alternatives to OLYMPIC" which asked field commanders to review their own plans in light of the Japanese build up on Kyushu and prepare plans for alternative operations. At the same time, King was also revisiting the concept of Olympic as he had previously warned the Joint Chiefs he would do in August. MacArthur on the other hand was gung-ho for staying the course of Olympic and played down the intelligence, claiming it was grossly exaggerated. MacArthur was known to exaggerate himself when he wanted his way and among senior American officers his judgement was known to be suspect.

Ultimately, had the war persisted then by late August Truman was likely going to be presented with dissenting arguments from both the USAAF and the USN against an invasion, as well as a JCS that was becoming somewhat wobbly on the whole idea as well. While Truman wanted the war over, the idea that he would have forced a costly plan for its conclusion in the face of opposition from the Navy and the Army Air Force, particularly when they casualties of that invasion could have doubled the American war-dead, is not very credible. And while a longer war might not have been desired, it was certainly not beyond America's capability to fight.

All of this brewing drama was nipped in the bud by the Japanese surrender but Frank lays out a very clear argument that as of early August the consensus agreement on Olympic and invasion was swiftly unraveling.
 
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...

Ultimately, had the war persisted then by late August Truman was likely going to be presented with dissenting arguments from both the USAAF and the USN against an invasion, as well as a JCS that was becoming somewhat wobbly on the whole idea as well. While Truman wanted the war over, the idea that he would have forced a costly plan for its conclusion in the face of opposition from the Navy and the Army Air Force, particularly when they casualties of that invasion could have doubled the American war-dead, is not very credible. And while a longer war might not have been desired, it was certainly not beyond America's capability to fight.

All of this brewing drama was nipped in the bud by the Japanese surrender but Frank lays out a very clear argument that as of early August the consensus agreement on Olympic and invasion was swiftly unraveling.

There is also the question of MacArthur reducing the estimates presented by his Medical staff & those from Krugers staff. Had he been outted on this & the other Army estimates closely examined at Marshals level a further loss of support in the War Dept & Washington in general would occur.

The opinions of Kruger, Echelberger, & others directly involved in the CORNET & OLYMPIC operations might be interesting.
 
There is also the question of MacArthur reducing the estimates presented by his Medical staff & those from Krugers staff. Had he been outted on this & the other Army estimates closely examined at Marshals level a further loss of support in the War Dept & Washington in general would occur.

Quite. The initial estimates presented to Truman in April of 1945 (which was something on the order of 40-50,000 dead instead of the ~300,000 minimum they had become several months later) were woefully out of date by August of 1945, something MacArthur had concealed from Truman. The Navy had found out though and they were quite transparently preparing to blow the whistle on him. Once Truman was presented higher assessments for American casualties backed by solid intelligence (it's notable that he was never shown firm numbers at the time and most of his recollection on the projected losses has been shown to have come post-war), the only way he would have authorized it is if he had 100% full consensus of his military leaders. And the Navy's and USAAF's imminent revolt would mean he wouldn't have that.
 
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One of the problems with the projected campaign and losses is it would be a winter campaign. Japan is not a tropical island. It is cold there in the winter, with cold rain in the south, and snow in the north, or snow up any mountain ridge above a few hundred meters. The infantry regiments are going to have loss rates from frostbite, hypothermia, pneumonia, trench foot, ect.. comparable to the US Army in Italy in the winter of 1943-44.

Then there is combat fatigue, which increases combat casualties.

Then there is a question of MacArthurs reputation for under preparing the logistics side of his campaigns. Echielberger & Kruger fought a uphill battle against this in the S Pac & Phillipines campaigns. Are shortages of ammunition, tents, mines, stoves, medical supplies, and more ammunition going to increase casualties?
 
The projected or estimated losses reflected internal military politics. Mac was anxious to get on with a invasion, so he argued it would be easy. The Admirals saw no reason for a land campaign and were arguing for a naval blockade. So, they went high on their projections.
How ready do you think the American public was for more casualties, even at MacArthur's lower estimate? Was there ever any polling taken to indicate whether the public wanted to "crush them now" or instead let them wither away?
the US Army Air Force was also poised to come out against the invasion. On 11 August, following the receipt of a comprehensive study on the effects of bombing in Germany, Spaatz had issued a new targeting directive turning the bombers away from firebombing of population centres and instead aiming them at Japan's transportation network.
Suppose Truman at least postpones DOWNFALL to allow Spaatz's new directive to take effect; how long is he willing to wait? Would the specter of a Soviet Hokkaido shorten the timetable, or would that be considered a small price to pay for the savings of American lives and materiel?
 
Suppose Truman at least postpones DOWNFALL to allow Spaatz's new directive to take effect; how long is he willing to wait?

Probably at least into next spring or summer. The effects on Japan, however, would have been positively apocalyptic.

Would the specter of a Soviet Hokkaido shorten the timetable, or would that be considered a small price to pay for the savings of American lives and materiel?

Assuming a Soviet invasion of Hokkaido is carried out and successful (it might, it might not)… that's hard to say. The US armed forces appears to have been under the general impression that the USSR might make a go at Hokkaido, Halsey was under the impression that his July carrier air strikes against the Tsugaru straits ferries was to assist a future Soviet assault, but there doesn't seem to have been any thought given to it at the political level.
 
Operation HULU put a fair number of assault landing craft in Soviet hands. OTL they used them in a series of small & mostly successful landings on the Korean coast. How that might have worked against Hokaiddo I cant say. As on Okinawa or Kyushu there were a lot of armed citizens on the muster, but not so many in regular trained army formations.
 
Operation HULU put a fair number of assault landing craft in Soviet hands. OTL they used them in a series of small & mostly successful landings on the Korean coast.
Very interesting. If the war had dragged out to Spring 1946, do you have any idea of the type and number of ships the Soviets would have had for Pacific operations? I imagine that most would never have returned to American hands once US-Soviet relations inevitably deteriorate after Japan surrenders or simply collapses.
The US armed forces appears to have been under the general impression that the USSR might make a go at Hokkaido, Halsey was under the impression that his July carrier air strikes against the Tsugaru straits ferries was to assist a future Soviet assault, but there doesn't seem to have been any thought given to it at the political level.
When Stalin told Truman he was ready to attack Japan in August 1945, did Stalin ever indicate the ultimate limits of his invasion plans? Had the US military broken any Soviet codes that could give them at least a short term warning of Red Army objectives?
 
When Stalin told Truman he was ready to attack Japan in August 1945, did Stalin ever indicate the ultimate limits of his invasion plans?

Not prior to the surrender. Stalin always played a close chest with his allies and was always relatively vague with the timing and location of any attacks. Among the Soviet general staff he indicated that preparations for an attack on Hokkaido should go forward, but those preparations were then aborted due to Japan's surrender and heavier then expected resistance in the Kuriles. The closest he came was making an inquiry with Truman about a Soviet occupation zone in Hokkaido, which Truman rebuffed, but that was after Japan's surrender and was discussing occupation... not invasion.

Had the US military broken any Soviet codes that could give them at least a short term warning of Red Army objectives?

Nothing at that high a level, to my knowledge. There was a program underway to try and get into NKVD/GRU high level encryption but it wouldn't start to bear fruit until 1947. They could, of course, try and track radio traffic at the lower levels and construct a picture from analysis of that but that would require putting SIGINT ships in the Sea of Japan.
 
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Supposedly the Lektor device was 'highly inspired' by the German T52d, and 'somehow' no one has ever (officially) found a T43 outside of photographs. Japan may well have had access to the former, maybe even the latter.
 
Supposedly the Lektor device was 'highly inspired' by the German T52d, and 'somehow' no one has ever (officially) found a T43 outside of photographs. Japan may well have had access to the former, maybe even the latter.

Uh, kind of short on context there. Can you elaborate our link some background reading on that?
 
Uh, kind of short on context there. Can you elaborate our link some background reading on that?

https://en.wikipedia.org/wiki/Siemens_and_Halske_T52, or better http://users.telenet.be/d.rijmenants/en/focus.htm, or even better http://www.cryptomuseum.com/crypto/siemens/t52/index.htm

http://www.cryptomuseum.com/crypto/siemens/t43/index.htm, or better http://www.cryptomuseum.com/crypto/stk/etcrrm/index.htm

Japanese possession of the advanced German cypher machines is trickier to document...
 
First things first. The USA had no moral requirement to protect the lives of the Japanese people. They started it. The USA finished it. The helpless Japanese people were an excellent example of what happens to a national population when the occupants, (citizens) lose control of their controlling government. Also strongly consider that the Japanese people for over 500 years were most effectively and completely disarmed from weapons. All weapons.

The two, (2) A-bombs were used to get Japan to surrender quickly. To end it. The main rational used by the USA commanding military was that if the USA had invaded the actual Home Islands of Japan it might, (probably) would have cost the USA up to ONE MILLION dead and wounded. Had the civilian Japanese population completely resisted and many thought they would, it may have cost 10 to 20 million Japanese civilian dead and wounded.
 
IIRC, something like 13% of all people in America at the end of the war would have supported an overtly genocidal campaign in Japan, but I can't find the source for that to save my life, but it shows that at least among some an invasion of the Home Islands would be worth it, even if it was brutal and bloody.
 
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