This is how esteemed moral theologian Fr Heribert Jone dealt with the morality of the use of nuclear weapons. If one looks at the actual circumstances within which Truman made the decision to drop the atomic bombs on Japan through the lens of what Fr Jone says, Truman’s decision fits perfectly within that framework.
The fourth condition required for positing an action that has an evil effect that there be a sufficient reason, i.e., a proportionate resulting good, to
permit the evil effect. The morality of using either the atomic or hydrogen bomb as a weapon of war is therefore, not a question of principle, which remains unchangeable, but a question of fact, and the fact questioned is whether there can be a military objective so vital to an enemy, the destruction of which would be a sufficient reason to permit the death of a vast number of civilians who at most contribute only remotely and indirectly to the war effort. We think this proportion can exist 1) because
today’s concept of “total war” has greatly restricted the meaning of the term “non-combatant”; 2) because in modern warfare the conscription of industry, as well as manpower, greatly extends the
effort on the home front; and 3) because it is difficult to set limits to the defense action of a people whose physical and even spiritual existence is threatened
by a godless tyranny. Therefore, while use of atomic weapons must be greatly restricted to the destruction of military objectives, nevertheless, it may be justified without doing violence to the
principle of a twofold effect. (Moral Theology #219
pp. 143-44 1961 Edition)”