My research into the two pertinent options, Operation Downfall and, even worse, the ongoing external blockade of the Japanese home islands plus the continued suppression of the transportation network on them, suggests that my counterintuitive proposition, that nuking Hiroshima was the least costly in terms of human misery, may be correct.
Documentation regarding the planning of Downfall's components, Olympic (the Kyushu landings) and Coronet (Honshu landings), shows that we would have seen continued bombardment of civilian populations, as well as--very probably--the use of chemical weapons to root out ensconced Japanese defenders. Additionally, while we likely vastly overestimated how many casualties we would suffer, but American and Commonwealth dead and wounded would have been significant. The Japanese civilian toll, if Okinawa is anything to go by, would have been ghastly.
But, who knows--they might have surrendered after a successful landing on Kyushu. It's possible.
As for the option of blockade, using ships, and particularly mines in the interisland waterways and bombers against "transportation targets" (you know, like Dresden

) the war would have continued for far longer. The economic collapse of Japan was already well underway. This, imo, would have been the most costly option of all, since the Japanese military would not have been under quite the same intensity pressure to surrender, but the civilian population would have been reduced to pure, abject starvation.
Ever seen Grave of the Fireflies? Multiply the duration times two or three. I mean, the movie made me want to kill myself after an hour and a half.
Plus, I think it's always important to remember that Japan had hundreds of thousands of soldiers in China, fighting the Chinese. Chinese military and civilian casualties and Japanese battle casualties cannot be overlooked in any analysis of whether a quicker if more violent end was preferable. Nor can the starvation of the bypassed Japanese island garrisons (Rabaul, with some 80,000 personnel iirc, had been sealed off for over two years by this point, again if iirc) be ignored in this kind of uncertain calculus.
Now, we could argue all day whether Nagasaki was necessary. I don't think it was, but I understand why it was undertaken. I'm also not convinced by the evidence of the proponents of the theory that the entry of the Soviet Union into the war was the decisive blow, not the nuclear bombings.
I think a Hiroshima Alone scenario would have been the most satisfactory of the plausible outcomes of the war.
As for the Cold War, why wouldn't we call the Cold War a time of stability? Europe had bled itself dry every thirty to fifty years for the previous... what, millennium? We're going on sixty-five years with no European war in sight. You can credit this to the unparalleled horror of World War II, but the external threat of the Soviet Union has, imo, quite a lot to do with it.
This sounds dismissive of the real tragedies produced by the Cold War in Korea and Vietnam and elsewhere, and I don't mean it to be--but a loss in Vietnam or even, perhaps, Korea was no occasion for nuclear warfare. A loss in West or East Germany would have been. And both sides knew it.
And, no, I don't think a cold war with China would be preferable to our current relationship. I have great qualms about that relationship, as I would with any close relationship between the U.S. and a dictatorial government that is feeling its way toward the edge of financial, political and demographic oblivion, but nothing that would prevent me from having warm feelings toward a people who are comprised in part, at least for now, of six hundred million Asian women.
Uh... also, Star Trek. Picard is better than Kirk. Right.
