Is the Atomic Bomb Immoral?
A replica of the “Fat Man” atomic bomb dropped on Hiroshima
The following essay is by Fr. Bernard de Lacoste Lareymondie of the Society of Saint Pius X.
On November 24, 2019, at the Hiroshima Peace Memorial, Pope Francis said: “I wish to reiterate with conviction that the use of atomic energy for military purposes is today more than ever a crime, not only against man and his dignity, but also against any possibility of a future in our common home. The use of atomic energy for military purposes is immoral as is the possession of atomic weapons.” 
Two days later, on the plane returning from Tokyo to Rome, Pope Francis replied to a journalist: “I repeated that the use of nuclear weapons is immoral - it must be inserted into the Catechism of the Catholic Church – and not only the use, but also the possession, because of an accident due to its possession, or the madness of a ruler, the madness of one alone can destroy humanity.” 
What to make of such claims? Already in 1965, during the discussions on the constitution Gaudium et Spes of the Second Vatican Council, theologians were arguing over this topic. Some wanted the Church to purely and simply condemn the use and possession of military nuclear weapons. Others, on the contrary, saw the atomic bomb as a means of self-defense.  In the end, the Council did not settle the debate.
In 1982, Pope John Paul II believed he could affirm that, “Under present conditions, deterrence based on balance, certainly not as an end in itself but as a stage on the road to progressive disarmament, can still be considered morally acceptable.”  However, this position was not enough to calm the controversies within the Church concerning this issue.
In December 1982, among the bishops of the United States of America, the differences were manifest. Some, like Joseph L. Bernardin, Archbishop of Chicago, wanted to call military nuclear weapons immoral. Others, such as the Archbishop of New Orleans, Philip Hannan, offered a more nuanced assessment, mentioning “the fact that we have a duty to defend Western Europe and that each of the nations that compose it wishes the presence of our nuclear weapons in Europe to ward off Soviet aggression.” 
How Can Catholic Moral Principles Be Applied to this Subject?
Let us first analyze the two arguments invoked by Pope Francis: the atomic bomb is immoral because it opposes the dignity of the human person and because it imperils the protection of our planet. For a Catholic, these arguments are very fragile and inconclusive. They draw on dubious philosophy and could lead to the moral prohibition of war itself.
St. Augustine, on the contrary, clearly showed that immorality does not reside in the fact of killing, but in injustice: “What is the evil in war? Is it the death of some who will soon die in any case, that others may live in peaceful subjection?”
“This is mere cowardly dislike, not any religious feeling. The real evils in war are love of violence, revengeful cruelty, fierce and implacable enmity, wild resistance, and the lust of power, and such like.” 
In the same spirit, Pope Pius XII, in 1948, denounced false pacifism. He condemned “the attitude of those who abhor war because of its horrors and its atrocities, its destruction and its consequences, but not also because of its injustice. This sentiment creates the fortune of the aggressor.” 
So let us put aside the false arguments of human dignity and ecology. In moral theology, it is first of all the natural and divine law that must guide men. But what does it prescribe?
According to the Fifth Commandment of God, it is never permissible to directly kill an innocent person. It is inherently bad. It is a mortal sin against justice. Therefore, even in the context of a just war, killing large numbers of civilians to pressure the enemy into surrender is seriously immoral. However, if it is a question of indirectly killing an innocent person, the question is more subtle. This is permitted under the following conditions:
- That the death of the innocent is not wanted, but only permitted and tolerated. 
- That the death of the innocent does not cause the desired good. St. Paul says in fact that it is not permissible to do evil in order to achieve good. 
- That there is a proportionate cause. 
It is this last condition that risks not being fulfilled in the case of the atomic bomb. For example, if, while bombing a large enemy military base, I indirectly and unwittingly kill two or three civilians, the proportionate cause is present. But if, in order to kill five enemy soldiers, I take the risk of causing the death of hundreds of civilians, the cause is not proportionate.
Now, the atomic bomb is very deadly. Its use will only be legal if the damage caused to civilians is very limited. It is for this reason that it is difficult to justify the bombings of Hiroshima and Nagasaki in August 1945.
But does this mean that the atomic bomb itself is immoral? Certainly not. The morality of a weapon does not come from its nature but from the use that men make of it. For example, morally, dynamite is in itself neither good nor bad. In contrast, the use of dynamite by human beings will necessarily be good or bad.
The same for the atomic bomb. It is true that it is much more deadly. But it is not the effectiveness of a weapon that makes it bad. It is obvious that an action of war always aims to be effective. 
The difficulty lies in the destructive effects of this bomb: they are terrible and difficult to control. Nevertheless, it is not impossible to imagine a situation in which the innocent victims of nuclear weapons would be few. These are cases where the enemy military objective is very clearly isolated.
For example, if a powerful enemy military base is located in the middle of a desert, or on a sparsely populated island in the Pacific Ocean, then, if the war is just, the use of an atomic bomb might be permitted morally, provided that the power of the bomb is proportionate, as far as possible, to the size of the target. This bomb could also legitimately be dropped on a squadron at sea, very far from the coast.
It must, however, be recognized that such a situation is not frequent, and that consequently, most of the time, the use of the atomic bomb is not justified, because of the lack of proportion between the death of the many innocent and the desired military result.
This is why Pope Pius XII, with the precision that is customary for him, expressed himself thus in his Speech to the 8th Assembly of the World Medical Association, on September 30, 1954: “Is modern total war, ABC warfare (atomic, biological, chemical) in particular, permitted in principle?”
“There can be no doubt, particularly in view of the horrors and immense suffering caused by modern warfare, that to initiate it without just cause (that is, without being imposed by an obvious and extremely serious injustice, otherwise unavoidable), constitutes an offense worthy of the most severe national and international sanctions.
“One cannot even in principle raise the question of the lawfulness of atomic, chemical, and bacteriological warfare, except in the case where it must be considered essential to defend oneself under the conditions indicated. Even then, however, every effort must be made to avoid it through international agreements or to place limits on its use that are clear and narrow enough so that its effects remain limited to the strict requirements of defense.”
“When the employment of this means results in such an extension of the evil that it is entirely beyond the control of man, its use must be rejected as immoral. Here it would no longer be a question of defense against injustice and the necessary safeguarding of legitimate possessions, but of the pure and simple annihilation of all human life within the radius of action. This is not permitted under any circumstances.”
Alas, we are therefore forced to note once again that Pope Francis’s theology is flawed. Military nuclear power is not in itself immoral. It is true, however, that the conditions to be met for its use to be just are such that, in practice, the use of the atomic bomb is very rarely morally permitted. But this conclusion is enough to make the possession of nuclear weapons licit.
 L’Osservatore Romano of Tuesday, December 3, 2019.
 Roberto de Mattei, Vatican II , p. 318.
 Message of Pope John Paul II for the 2nd extraordinary session of the United Nations General Assembly on disarmament on June 7, 1982.
 Speech by Bishop Philip Hannan, La documentation catholique, January 16, 1983.
 Saint Augustine, Contra Faustum, ch. 74.
 Pius XII, Radio Message to the World of December 24, 1948.
 Saint Thomas Aquinas, Summa Theologica, II-II, 64, 6.
 Rom. 3:8.
 Summa Theologica, II-II, 64, 7.
 This is explained by Fr. Michel Labourdette in his Grand Cours de Theologie Morale, t. X (charity), p. 349.