I saw the RSC play Oppenheimer this week. It closes today so I’m sorry I can’t encourage you to see it.
There were many themes and ‘messages’. For me, the overriding one was how it became almost impossible for the Manhattan Project team to turn back once they had invested so much of their lives in the quest to build an atomic bomb. The project had a momentum and an inevitability – political, scientific, human, emotional – that made it inconceivable for anyone to question the morality of what they were doing or what it might lead to. (The photo above shows Maj Gen Leslie R. Groves, Robert Oppenheimer, and others at the remains of the Trinity shot tower a few weeks after the first test bomb was exploded.)
There were two heartrending lines (I’m quoting from memory). First, someone comes to Oppenheimer and says, “At this rate we will win the war before we finish the bomb”, and he replies simply “I hope not” – knowing that an end to the war would remove all the purported moral justifications for the whole project (“if we don’t get it first then the Germans or the Russians will”, etc).
Later on there is a well-known debate about whether it will be enough to demonstrate the Trinity test bomb to the Japanese, to impress upon them the threat of atomic war without having to actually start it. As well as all the pragmatic/diplomatic/military reasons against this option (“they still won’t believe us; what if the test goes wrong”; etc.) one of the scientists presents the scientific rationale for dropping the bomb on Japan: that without a drop on a real target we will never know what the real scientific effects of the blast will be. In other words, we need to drop the bomb on human guinea pigs in a real city to conclude the scientific experiment.
In moral theology terms, you could say that the attempt to build the bomb became an ‘occasion of sin’ for all those involved: it put them in a position where they were more likely to want to do wrong, and where their freedom to resist the wrongdoing became increasingly weaker. And therefore the decision to start the project was where the greater moral culpability lay. In other words, perhaps you can separate, theoretically, (i) building the bomb from (ii) using it; just as, theoretically, an alcoholic can separate (i) setting up a distillery in the basement and distilling a gallon of vodka from (ii) drinking it. But in practice the moment of walking down to the basement with the distillation equipment is the decisive moral turning point.
I rushed home to re-read the later sections of Max Hastings’s All Hell Let Loose: The World at War 1939-45. He writes about the ‘technological determinism’ built into the Manhattan Project:
Technological determinism is a prominent feature of modern warfare, and this was never more vividly manifested than in the exploitation of the power of atomic destruction. Just as it was almost inevitable that once an armada of B-29s had been constructed to attack Japan, they would be thus employed, so the United States’s commitment to the Manhattan Project precipitated the fate of Hiroshima and Nagasaki. Posterity sees the use of the atomic bombs in isolation; yet in the minds of most of the politicians and generals privy to the secret, these first nuclear weapons offered merely a dramatic increase in the efficiency of the air attacks already being carried out by LeMay’s Superfortresses, and provoked negligible expressions of moral scruple back home. 
Hastings continues later:
The atomic enterprise had a momentum of its own, which only two developments might have checked. First, Truman could have shown extraordinary enlightenment, and decreed that the bomb was too terrible to be employed; more plausibly, the Japanese might have offered their unconditional surrender. Yet through mid-summer 1945 intercepted secret cable traffic, as well as Tokyo’s public pronouncements, showed obdurate Japanese rejection of such a course. 
Thank goodness the rise of the SNP at the General Election has put the question of the UK’s Trident nuclear weapons higher up the political agenda; admittedly, still not very high, but at least higher than it was before.