Leo Szilard was one of the great physicists of the 20th Century, and played a critical role in the Mahatten Project. But he strongly opposed the use of the atom bomb on Japanese cities, not only because he objected to the mass slaughter of civilians, but because he (quite correctly) predicted that the actual use of the weapon would lead to a nuclear arms race with Russia, raising the very real and disturbing possibility of the destruction of human civilization itself.
In a 1960 interview, Szilard put forward a question that every American should ask themselves whenever they consider the moral implications of our use of nuclear weapons against Japan in 1945.
"Suppose Germany had developed two bombs before we had any bombs. And suppose Germany had dropped one bomb on, say, Rochester and the other on Buffalo, and then having run out of bombs she would have lost the war. Can anyone doubt that we would have defined the dropping of atomic bombs on cities as a war crime, and that we would have sentenced the Germans who were guilty of this crime to death at Nurnberg and hanged them?"
It's a good question.