A more realistic perspective is that pacifism (even "realistic") is a form of deontological cowardice that abdicates the moral burden of making difficult tradeoffs to more mature actors.

Is this more realistic or more credulous? Even in hindsight your analysis is extreme - before the bombs dropped the Japanese war council was aware of the looming Soviet invasion and subsequent Allied plans, and despite this its three non-military members were pushing hard for Soviet mediation and an end to the fighting - the war council was certainly too prideful and contentious to accept a quiet surrender, but the concern was over unconditional surrender to the West, not the idea of a negotiated surrender, which even the military members were open to in principle, before the Soviet ground invasion had begun. Many historians today believe that Japan would have surrendered on some terms before the planned November invasion, and most believe that had it not, the invasion would have forced a surrender.

You argue that there was no real chance of the bomb not being built, since the race was on against Germany and Japan. But Rotblat left the project in December of 1944! His direct motivation was the success of the Alsos mission, which concluded that there was no threat of a German bomb. Japan had never been close, lacking both the materials and scientists of sufficient quality, and there are no contemporary concerns about a race against Japan. The Soviet effort only began in earnest after the bomb dropped in Hiroshima, and only succeeded as quickly as it did because the Soviets received enormous amounts of information from Klaus Fuchs and Ted Hall, that is, only because the Manhattan Project proceeded as quickly as it did, and because the Soviets overturned the entire Eastern Bloc looking for uranium deposits, knowing that if they did not become a nuclear power they would not become a power at all. The other Allied powers outside of the USSR had sent their best scientists to the Manhattan Project - they were known not to have independent nuclear programs.

All of that is to say: when Rotblat left Los Alamos, before the bombs had been built, it was with the full and correct knowledge that no other nation in the world was close to building a bomb. At the end of the war, only one other nation in the world was close to building a bomb, and it was a direct result of the decision to push ahead at the maximum possible pace and to make a show of force.

You argue that Truman was faced with a "terrible decision" and made a principled, game-theoretic decision to drop the bomb and end the war quickly. But top military officials at the time disagreed explicitly - Leslie Groves famously said that there was no question that the bomb, once build, would be used. The motivations are not so clean as you'd like them to be either. Indeed SoS James Byrnes told James Forrestal and Leo Szilard that dropping the bomb would be advantageous because it would end the war through American might on American terms, rather than depending on the Soviet invasion. It's very hard to read the words of decision makers at the time and conclude that the bomb was dropped because it was the simplest, best, or only way to end the war. It's very clear that politics were a live consideration: Japan would fight on, but it would lose, and it was important to Truman and his advisors that Japan lose to America, not to Stalin.

And lastly you hagiographize the "wise politicians and army officers" who developed the political circumstances leading to nuclear deadlock and avoiding large-scale nuclear warfare after the initial bombs were dropped. Were they so wise, so heroic, as you claim? Kissinger, the caretaker of US military strategy for the latter decades of the 20th century, built his career advocating for tactical nuclear strikes, seemingly because he know that advocating for such a provocative position would make him stand out among academics in international strategy. Kennedy campaigned on a missile gap, stoking domestic fear of a nuclear holocaust and escalating tensions for political gain, but he knew that this claim was false. On the Soviet side, Khrushchev lied about the USSR's missile production, acting as a counterparty for the missile-gap myth, again because escalation was more politically expedient than seeming weak or behind on the world stage. The Soviets built an automatic retaliation system in 1985, for God's sake. Vasily Arkhipov and Stanislav Petrov, in acts of individual integrity, did not push the buttons that might have destroyed the world.

In hindsight bad outcomes will often seem inevitable, and it will be the job of decision-makers to convince you that they really were, that they acted nobly and made the best of impossible choices. When they are wrong, they will argue that the frenzy of the time, the general sense of things, meant that security-minded people could not risk de-escalation. This may sometimes be true.

In the modern day, after humanity has survived the nuclear arms race, we've found time for deproliferation, for compacts and guarantees. Even in a world where some countries have nuclear weapons and others don't, we're able to make treaties with hostile, nuclear-curious powers and use diplomacy to ensure that they don't try to build a bomb, so long as we're able to ensure that they will never need one.

Are you really so sure that these treaties could not possibly have been signed in a world where the Manhattan project was stopped, in a world where the US didn't develop the blueprints of total destruction and so there were no blueprints for spies to hand off to the USSR, in a world where Stalin could have believed that his regime might survive without developing the bomb in half a decade? Are you really so sure that the route we took, a route in which multiple individual soldiers, whose only qualifications were finding their way into a tense situation, had to disobey orders and be willing to sacrifice their lives if they were wrong? The future of the human race is not supposed to be in the hands of one person at a time, and it's not supposed to be passed around from individual to individual like a game of hot potato, but it was.

Are you really so sure that the politicians who oversaw the latter half of the 20th century were wise, were altruistic, were good? We know that so many of them were motivated by politics rather than by safety. We know that they lied and bluffed and escalated again and again, that they made decisions about what to build, when to act, and even whether to drop the bombs, with an eye towards their relative status and power in the world, rather than just the existence of a world at all. Over and over and over again, your analysis suggests, a thousand times and in a thousand ways, they made the right decision for the wrong reasons.

The pacifist commitment, really, is to refuse to be trapped in this sort of insanity. It's a known defect of human thinking that when we feel threatened, when we feel unsafe, we are unable to accurately process evidence to the contrary. Rotblat was not immune to this defect, certainly. Having heard the reports about the failure of the German nuclear program, could he really have been sure that he wasn't being mislead? Could he have been sure that the Soviets weren't, somehow, further along than they seemed to be? A million doubts must have leapt to mind. They clearly did for the rest of the Manhattan project. The leaders of the project and of the country had correct, reliable evidence that if they didn't finish the bomb, if they slowed down, there was no opponent right on their tails. They knew this, knew that their original reasoning for pursuing the project at this pace was no longer supported by evidence. But they were not pacifists. They were afraid, they were close, and they had not decided in advance to fight their fear, and so they continued. Of course they justified this to themselves and to the public. They argued that there was no real choice, that they were in a race against time, against the nation's enemies. Someone had to build it. Someone had to, you see, sooner or later, there's nothing that could have happened between the sooner and the later. And someone had to use it. It wasn't about ego, it wasn't about politics, it wasn't about fear. Sometimes you have to make hard decisions.

They were wrong, of course, about the facts. They knew it and Rotblat knew it. He put reason before fear and walked away, and they did not. They pursued the sort of power that would make them feel safe, even at the expense of the entire world, and they argued as fearsomely as a person can that they had to do it, that it was noble and good and right. And because they were pursuing power, and because they achieved it, they were able to make this argument as loudly and as officially as it is possible to make an argument, and you, it seems, believe them.

But what do you believe, really? Do you believe that they were right about the facts? Of course you don't. You argue that the bomb was inevitable, but they didn't just claim it was inevitable, they claimed that it was inevitable and impending, despite knowing this to be false. Do you believe they were right in their motivations? You claim to, but I don't think that's correct - as I've explained, while one might argue that game theory suggests that a moral actor would pursue their chosen strategies, there isn't any historical question as to whether the people in question were moral actors in that sense. A moral actor might have pursued an arms deadlock, but would not have lied about the number of arms in question, would not have pushed their side and the other deeper in tension for the sake of winning an election.

What you really seem to believe is that they were "mature". That pacifism is weakness, is cowardice, an abdication of responsibility. That any time a person believes that difficult tradeoffs might be in play, they are obligated to pursue them and reify them, to refuse to step back or consider reframings. Blinded by fear and ambition, the forefathers of the Cold War refused to acknowledge the intelligence in front of them. They had all signed on to build a bomb, believing that it was necessary to deter the Germans. Once that motivation faded, they had spent years in a wartime environment, and most of them failed to realize that they had been wrong, that the thing they had been working to mitigate, a nuke in Hitler's hands, was simply not going to happen. But these, you're sure, were the mature, rational actors.

Joseph Rotblat, because of his pacifism, refused to work on a military project whose object was not immediate and concrete, was as nebulous as "something something game theory." He noticed, correctly, that the race was off, when nobody around him did. He predicted, correctly, that racing ahead at breakneck speed would lead to conflict for its own sake, where both sides escalate because they feel they cannot afford to de-escalate, to the sort of recursive conflict where both parties believe they must be willing to risk everything because everything at stake, the sort of conflict that can only be cut off at the first step. He believed, correctly, that people stuck in these sorts of conflicts will have trouble evaluating all of their incentives and all of their options, will refuse to look for a way out and will instead follow their immediate incentives no matter the cost, and will justify this on strategic grounds. Pacifism has never been a particularly popular strategy, because credulous people tend to believe these arguments, they feel smart and smug for "realizing" that actually bad things are not avoidable. When bad things don't happen, you see, they don't happen, and so it can feel like wisdom to point out that nobody has ever observed a bad thing not happening, that any ideology which recognizes the failures of human rationality and refuses on principle to enter into cycles such as these has never been observed to work.

Joseph Rotblat was right about everything. The decision-makers you praise were wrong. Nobody can know whether or when nuclear arms would have been developed if the Manhattan Project had been prematurely ended in November 1944, but we can know that Fuchs wouldn't have given the Soviets a full working blueprint for Fat Man in 1945 if Fat Man had not been built, we can know that Stalin wouldn't have made developing the bomb his top military priority in response to the bomb being dropped on Hiroshima if the bomb hadn't been dropped on Hiroshima. Rotblat predicted that working on a weapon without a concrete need to do so, that is on non-pacifist grounds, would escalate conflict, and he was right. But he, you are sure, was really a "deontological coward" for this. Those who failed or refused to recognize this truth were the mature actors. Because, you see, they predicted that the invention of the nuclear bomb would bring about a new and terrible age in warfare in which anybody without a bomb could not ultimately stand up to anybody with a bomb, and then they invented the nuclear bomb, and then that did happen. What stronger evidence of maturity, than to shoot before any of your opponents are armed, and thus ensure that at least you don't get shot first? That's not paranoid, that's not pathetic, that's not evil or selfish, and it's certainly not cowardice. That's game theory. Cowardice would be doing anything else.

(EDIT: I should probably note that this is too strong on the invective, and apologize for that. You express some ambivalence at the beginning of your post, but the rest seems to have a lot more conviction, and I'm unsure about your actual stance here. This post is argumentative because I think the high-conviction version of your argument is very, very, very stupid and like 70% as evil, and I don't think it's possible for me to express the depth of my surety in a more straightforward way. I did at least manage to not blame Ayn Rand in particular for the existence of such evil and stupid arguments in the world, so I hope that I at least stayed on-message. I am open to the possibility of you simply not knowing these historical facts, or knowing them but thinking this argument is worth presenting without being attached to it, or even to knowing historical facts that would change my position, although I doubt this last. In any of these cases, I apologize that the tone of this post is more directed at you than it is supposed to be.)