But I still struggle to understand the Japanese mentality. Were they OK with the prospect of city after city being atom- or fire-bombed, so long as no ground invasion occurred?
The sad thing is that there is a non-zero chance they where right. There was considerable concern about that from the war leadership at this point. The allies _dramatically_ underestimated the forces that the Japanese had marshalled at the two invasion sites, and it would have been not very good. The alternative that the Navy was pushing was a starvation blockade of Japan. This probably would have been succesful but led to millions more lives lost in Japan, and a almost inevitable civil war in Japan.
They did not underestimate the Japanese forces.
They did an assessment in early 1945 and did calculations and the needed invasion force. But the Japanese could read a map just as well as the Americans, and could guess where Olympic would happen and redeployed, which the Americans detected:
* https://en.wikipedia.org/wiki/Operation_Downfall#Ground_thre...
For survivors of a city destroyed overnight, with a great many friends and relatives dead, others injured or suffering the follow on results of malnutrition, disease, etc. there's little to separate the ethics of the use of an atomic weapon from the ethics of the use of tonnes of HE and incendiary weapons.
The most famous comparison, Tokyo Vs Hiroshima, has little to distinguish them from the PoV of a survivor.
https://blog.nuclearsecrecy.com/2014/09/22/tokyo-hiroshima/
Again, for context, 72 Japanese cities, including Tokyo, were each leveled in single bombing raids before Hiroshima.
Any debate about the killing of civilians to "save US soldiers" from invaders predated the use of atomic weapons by a number of months when the policy to flatten all of Japan from the air first started.
Atomic weapons were developed for use against Germany, with the German surrender prior to a field testable weapon being built, with the imminent collapse of Japan on the horizon, it was a race by those military at the head of the Manhattan project to have clean field targets before the conventional bombers worked through their list.
Hiroshima was to be destroyed by US forces either way.
The question should be less about the ethics of spending an ungodly amount of money to destroy a city with a single bomb, and more about the ethics of destroying a city for considerably less cost with multiple bombs in a mass raid.
I very much suspect a great many regular people in Japan were very much not okay with seeing their fellow citizens destroyed in a war being pursued by other elements of Japanese society.
The historical record is very clear, as is Hirohito's own statement at the time:
"Furthermore, the enemy has begun to employ a new and cruel bomb, causing immense and indiscriminate destruction, the extent of which is beyond all estimation. Should we continue to fight, not only would it result in the ultimate collapse and obliteration of the Japanese nation, but it would also lead to the total extinction of human civilization.
How, then, are We to protect the millions of Our subjects and atone before the spirits of my Imperial ancestors?
This is why We have ordered the Empire to accept the terms of the Joint Declaration. "
I really strongly recommend Ian Toll's histories of the Pacific war, because there is information that is not in the early histories (at least the ones I read a while back) that is because things in the US and former USSR are more declassified. In particular, the US had a pretty good understanding of what is going on in the Japanese government. They knew a lot of civilian leaders wanted to surrender before Hiroshima, but were afraid they would be associated by a coup, as had happened in the 1930s. That's also what the Japanese leaders who wanted to surrender were worried about as well.
thats just the official word. The coming Soviet invasion was definitely a factor.
In comparison, the proposed allied invasion was planned to have 42 aircraft carriers, 24 battleships, and 400 destroyers and destroyer escorts.
"Now that the Soviet Union has entered the war against us, to continue the war under the present internal and external conditions would be only to increase needlessly the ravages of war finally to the point of endangering the very foundation of the Empire's existence."
Hirohito said different things to different audiences.
He was a war criminal, but not the leader of the war by any means. That was reserved for the militarists - Tojo and Suzuki and others.
That's definitely enough when you are a living god. From the point he gave his approval, he was the only one that could fully stop it.
You're right that he was a puppet for the military, exactly because of that god position he was born into.
Depending on how you look at it (and reams of paper have been expended on this topic), he was somewhere between a puppet and a symbol. Certainly not innocent, but also not the instigator.