Now of course there has long been controversy about Truman's decision to drop the atomic bomb 75 years ago today. It is hard to escape the conclusion that it did help speed up the end of the war (along with the Soviet Union's entry into the war three days later). Historians will argue endlessly about just how decisive it was. Would Japan have given up soon anyway, especially given the Soviet intervention, or would the fanaticism, that even after both the bomb and the Soviet entry still tried to prevent the Emperor's surrender broadcast, have kept the Japanese fighting to the end at great cost to both sets of belligerents? There are ultimately unresolvable historical questions. A more manageable question is whether the successful slogan but otherwise poor policy of "unconditional surrender" got in the way of ending the war earlier. (That was my view back in college.) In fact, "unconditional surrender" was somewhat selectively applied - absolutely in the case of defeated Germany, but certainly more moderately in the case of Italy and at the end in a somewhat attenuated way with Japan, insofar as the imperial throne was preserved, which by that point was Japan's main condition anyway.
Quantitatively, the bombing of Hiroshima was less destructive than the conventional bombing of Tokyo had been. Controversial even at the time, the Allied policy of massive bombing of civilian cities was morally problematic for many of the same reasons offered by those who criticize the use of the atomic bomb. But I think the critics of the atomic bomb do make an important point in that it was qualitatively different. That one single bomb could inflict so much damage - including long-term radiation damage not fully appreciated in advance - does make atomic weapons (and, a fortiori, the even more destructive hydrogen bombs, etc., that followed) qualitatively different in a way which warrants special horror and concern. The subsequent arms race, as a result of which it became possible to imagine "Mutually Assured Destruction," did in fact create a qualitatively different moral context.
I grew up during the Cold War and can well remember the famous, semi-annual drills, when we would all run and hide or, if in school, duck under our desks. One of the paradoxes of that experience was how we simultaneously could imagine the end of civilization as we knew it and at the same time continued to go about our ordinary lives - as Jesus said, as it as in the days of Noah! That paradox reflected the success of nuclear weapons as a deterrent. In fact, nuclear weapons not only successfully deterred either side from starting a nuclear war, but the legitimate fear of escalation also successfully deterred either side from starting a full-scale conventional war in post-war Europe. Such a war would almost certainly have occurred, had it not been for the fact that both sides possessed nuclear weapons. And, had such a war occurred, it would likely have surpassed World War II in its toll on human life and civilization.
It is still the case that deterrence works in that it is infinitesimally unlikely that either the U.S. or Russia would ever deliberately launch its nuclear missiles. Proliferation has not changed that calculus. No other nuclear-armed state has used its weapons so far.
However, the possibilities of accidentally starting a nuclear conflict have increased as more actors have access to such weapons. It was the real danger of war by mistake -"miscalculation" as it was usually termed - that loomed most dangerously over the Cold War and still does today. Add to that the increased possibility of unstable or rogue actors acquiring such weapons and the likelihood of accident, mistake, or miscalculation increases considerably.
If I could, I suppose I would rewrite history so that Archduke Franz Ferdinand's car did not take the wrong turn and end up in Gavrilo Princip's range on June 28, 1914. But history cannot be done over. Whatever might otherwise have been, the atomic age dawned 75 years ago today. Relitigating the rightness or wrongness of decisions made at that time is a political dead end.
On the other hand, reshaping or contemporary world order to protect human life and civilization from an accidental or mistaken or miscalculated resort to nuclear weapons ought to be much more of a conscious concern and priority than it presently is. Like pandemics, nuclear war is only a theoretical problem - until suddenly it ceases to be just theoretical, at which point it may be too late. If anything the social stresses and security threats posed by climate change and future pandemics make international accidents, mistakes, and miscalculations that much more likely. The same dangerous insouciance that has brought us to this present pandemic disaster and the increasingly proximate climate change disaster should stand as a warning not to keep doing the same with nuclear weapons as well.