Returning to the debate about dropping the bomb on Japan
The release of Oppenheimer has re-opened the recurring debate about the use of nuclear weapons on Japan. Was it necessary, atrocity or both? What other options were there. The question is examined both in full hindsight as well as considering it in the context of what they knew then.
You can read lots of analysis of the different arguments in many sources. There are arguments that Japan was already going to surrender, or that they surrendered more because of the Soviet invasion of Manchuria than the nukes, or would have surrendered without Nagasaki, or with just a demonstration as was proposed for Tokyo Bay by many of the scientists (but rejected by Oppenheimer.) There is the "standard" view, promoted by Truman and many others, that it was necessary to avoid a more ruinous invasion and the "shock and awe" to use our modern term allowed them to (in the words of Hirohito) consider the unthinkable. There is evidence for all the views.
I've been exploring some thoughts that are much less commonly discussed, which is not to say that they are strongly supported, but that they are interesting. These include the idea that the main value of the bombings, and possibly at least in part their intention, was to scare Stalin, not the Japanese. There are also a number of arguments why they should have bombed (or demonstrated) sooner, possibly much sooner, and some amazing and chaotic accidents of history and the timelines of the day. It is both an amazing coincidence and no accident that Stalin began his invasion literally a few hours before Nagasaki, and this invasion changed the course of world history in astonishing ways.
Whatever you think about the timing of Hiroshima and Nagasaki, it seems that had the allies had the bomb far earlier -- including years earlier, there would not be much debate over it. Defeating Hitler in 1944 or 1943 prevents a large fraction of the Holocaust. It prevents both the death of many millions of Soviets, allied soldiers and civilians and huge numbers of Germans. It prevents Hitler's scorched earth plan and the destruction of countless European towns by both the Axis and the Allies. As much as the destruction of Berlin (particularly while Hitler was there) would have been a horror, it is a vastly smaller horror than what happened without it. The Japanese would probably have also sued for peace as well.
On top of all that, done early enough, Stalin would not have marched through so much of Eastern Europe, occupying and giving 45 years of ruin to so many millions -- and murdering many millions more, possibly. There would not have been an East Germany ruled by Russia. Go early enough and even Poland is not occupied. At least in 1945 -- Stalin did expand later (and acquired his own nukes) in the original history. People estimate Stalin's deliberate victims at close to 9 million, including those he deliberately starved.
So, had the allies had a nuclear weapon in 1943 or 1944, one would hope there would be little debate about the morality of its use. It's not the ends justifying the means, but rather the vastly lesser of two evils (as the standard view of Japan argues.)
This leads to an interesting counter-factual question. What would Stalin have done in east Asia, given more time? We saw what he did in Eastern Europe. And indeed, we saw that with just a very short amount of time before the full Japanese surrender, he conquered Manchuria and other parts of China, and North Korea. He set up a puppet state in North Korea that of course led to the Korean war and one of the world's most evil states today which keeps millions in slavery and is the most unstable nuclear power. This was no accident -- the USSR had been grooming Kim Il-Sung for some time and he was in the Soviet Red Army and Stalin installed him as ruler after their conquest of the area. Presumably you know the rest of the history of him and his family.
Mao wasn't much at the end of the war. The CCP controlled minimal territory. The Soviet conquest of northern China allowed them to hand control of it to the CCP, which turned the tide, a large factor in powering the communist revolution which pushed the RoC out to Taiwan. Of course, Chiang Kai-Shek was an autocrat and no prince and that situation was complex, though his successors took the path to democracy and prosperity.
All of this happened even with the bombing of Japan. At Potsdam, an agreement was made with Stalin that he would enter the war in Japan by 3 months after the fall of Germany. Here things get murky -- he did wait the full 3 months. Prior to entering he was playing games with the Japanese, pretending he might not enter the war. They even tried to negotiate a peace deal with him. The key "revisionist" thesis about the war was that the Japanese surrendered not because of the nukes, but because they knew they were truly lost with the Soviets in the fight. Indeed, Stalin was (truly) a much scarier opponent and occupier than the Americans. Truman knew that and there is evidence he used that as a lever as well. In the event of an invasion, we would have probably seen a "North Japan" similar to "East Germany" at the end of it, and neither Truman nor the Japanese wanted that, but it was a scary threat.
Even had Japan surrendered without nuclear attack, it seems likely we would have seen a much larger Soviet iron curtain in Asia to match what we got in Europe. Which would have been bad for everybody, and in particular the Japanese. Possibly the South Koreans and some other countries as well. The 20th century would have been quite different. The Korean and Vietnamese wars would not be as we knew them. It's hard to be sure.
Roosevelt's, and later Truman's views on Stalin changed over this period. They went back and forth on what the Soviet involvement would be in Asia. The strongest force, of course, was their value as a powerful and scary force, reducing burden on the USA. Until the atomic bomb, the idea of the USA doing this on its own wasn't at all likely. To get the USSR to enter the war, it had to be given concessions. By V-J day, there was regret that they were coming in.
An interesting hindsight counter-factual is that in July of 1945, the USA had a 3rd nuclear weapon, which we know worked, but they didn't know that and so they wanted to test it at the Trinity site. This weapon could, in theory, have been used on a Japanese city, or in a demonstration -- followed up shortly after with the other two bombs if Japan did not capitulate. It is argued that the 2 bombs used were spaced too closely. Truman did not plan that and was surprised, the date of Kokura/Nagasaki was moved up 2 days due to weather reports. The use of Gadget and Little Boy with a bit more spacing might well have done the job -- and also eliminated the need for Soviet involvement.
Or not. Stalin reportedly sped up his invasion after Hiroshima. He wanted in before the war ended. Perhaps he would have come in, even if the war had ended in July, though perhaps that would have been too fast, or at least too fast to get as far as he did. The Japanese formal surrender was not until Sept 2, and Stalin took advantage of that. Americans with perfect foresight would have perhaps pushed for more speed. It would have been a slight abrogation of their deal with Stalin to enter the war in exchange for control of Manchuria and return of a path to the Pacific. By this time the USA wanted out of that deal.
The final consideration, which has been discussed before but which I think is true, is that the horror of Hiroshima and Nagasaki created a different view of nuclear weapons in the world which would not have come from their non-use or demonstration use. I grew up in a time when we all sort of assumed there would eventually be a nuclear war, but since Nagasaki they have never been used. The risk is not gone but it's vastly lower today, even with Russia's current actions. So far, the horror of those bombs seems to have worked. Today, for better or worse we have more "useful" weapons and we will make ones that are more useful still, and may come to a time when the idea of using something as crude as a nuke will seem silly, putting them forever behind us, though whatever creates that sentiment will probably be pretty scary too.