5

Clearly the Japanese believed that if they did not surrender another atomic bomb would be used on them.

However how soon did they believe this would happen?
And how many atomic bombs did they believe the USA would have access to?

  • 1
    Who do you mean by "the Japanese"? There was not really a unified opinion in Japan at the time. Without deeply researching the subject I'm aware that some regional authorities had distinctly different ideas about a third bombing from the imperial leadership, and the military. – Semaphore Aug 21 '15 at 12:43
  • do not forget that the Russians attacked. Suppose, e.g., that the Japanese thought that another A-bomb would not be ready in the next 6 months. Where would the Red Army be in 6 months? Korea? Hokkaido? The A-bomb and possible American invasion were not their only reason to surrender. – Luiz Jul 12 '18 at 19:42
  • @Luiz: "Where would the Red Army be in 6 months?" Without American help, Siberia? Did the Russians have any real naval force in the Pacific, or any way to get their army across the ocean and do a successful amphibious landing? – jamesqf Jul 13 '18 at 18:10
  • The Japanese kept a destroyer flotilla opposing the Soviet Navy, and that was much more powerful than anything the Soviets could muster in theater. The Soviets did do amphibious landings, and really sucked at it. The landing in Sakhalin was saved by the overland advance, and the landings in the Kuriles saved by the general surrender. – David Thornley Jul 13 '18 at 20:43
  • @james They did not need a navy to reach Korea. And they did have USA help. USA gave them landing crafts and even destroyers that the soviets used to invade the Kurilas. It is debatable if the soviets had enough naval resources to invade Hokaydo after the Kurilas, but at least they did some smaller scale landings against Japan - with American help that they already had in August 1945. Look in wiki about the Kurilas invasion and lend lease in pacific. The russians were trained in Alaska by the US Navy – Luiz Jul 15 '18 at 19:44
8

I think you are formulating the debate in the wrong terms.

There were Japanese who correctly believed that the war was lost, nukes or no nukes.

There also were Japanese, who believed that an honorable settlement was still possible, through some far fetched pipe dream scheme like Soviet mediation or Kamikaze pilot wild successes.

Nukes gave the former a decisive argument against the latter. It did not really matter how many more bombs the US had. What did matter was that the latter group of Japanese leaders could no longer deny that their near future involved a complete extermination.

6

The Japanese had no real idea how many atomic bombs might be available. They had not had any specific knowledge of the Manhattan Project. Their scientists were aware of the possibility that such bombs could be built, and they had a small atomic weapons research project (it didn't get anywhere).

Once the first bomb was dropped, their scientists realised what had happened within about two days, although explaining it to the military and politicians was harder.

Using the second bomb relatively quickly may have been intended to give the impression that there were plenty more. Another would have been ready for August 19th, with three more in September and three in October.

Richard B Frank's Downfall: The End of the Imperial Japanese Empire is a pretty good source on this period.

2

Wikipedia has some information on that. First of all, since the Japanese did have their own nuclear project, they knew that separating uranium was slow and costly, and (correctly) understood that the US would only have a few bombs. In fact, second bomb was dropped shortly after the first partly with a specific goal of making the impression that the US had a large supply.

However, on August 8th, a US pilot Marcus McDilda was captured and, under torture, claimed that the US had 100 bombs and is intending to use them soon (in reality, he knew nothing relevant). Apparently, this was taken seriously by the Japanese, at least the war minister Anami informed the cabinet, influencing the surrender discussions.

  • Re "separating uranium was slow an costly", did they know anything about plutonium? – jamesqf Jul 12 '18 at 16:59
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    well, according to wiki they only tried uranium, and with little success. But plutonium production is also not fast. – Kostya_I Jul 12 '18 at 20:25
  • @jamesqf Plutonium requires an implosion bomb, a significantly more complex device involving using explosives to form a spherical shockwave to crush a hollow plutonium sphere juuuust right and form a critical mass. That's what the Trinity Test was testing and what Fat Man was. In contrast, a uranium bomb like Little Boy is a matter of firing a slug of uranium into another slug of uranium to produce a critical mass. Inefficient but simple. So simple they didn't even bother testing it. I wouldn't be surprised if the Japanese assumed uranium because an implosion bomb was so daunting. – Schwern Jul 12 '18 at 20:35
  • But plutonium production is faster & easier than uranium separation, and they run in parallel. What I meant was that if the Japanese had based their estimation of US nuclear capability on the only uranium assumption, it'd fall well short of actuality. – jamesqf Jul 13 '18 at 16:26
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    @jamesqf: Nuclear weapon programs were very expensive, and Japan didn't have the resources. They'd be very unlikely to make progress beyond knowing what a U-235 bomb would be like, and running into some of the problems with uranium enrichment. Japanese scientists correctly thought the Hiroshima bomb was U-235, and they more-or-less correctly believed that it would take a long time, maybe a year, to make another one, The Nagasaki bomb was completely unpredicted. – David Thornley Jul 13 '18 at 20:40

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