Over the past several decades, the Catholic theory of “just war” has been trotted out, often in bits and pieces, to justify this conflict or that conflict, to explain this nation’s actions as morally permissible and that nation’s as atrocities. While Pope Leo XIV’s first encyclical, Magnifica Humanitas, is centered on the emergence of artificial intelligence (A.I.) in the modern world and its proper and moral usage, the first American pontiff may also be putting an end to the abuse of Catholic moral teaching as a justification for violence.
“We must acknowledge that, despite the desires and declarations for peace, the past sixty years have been marked by conflicts of astonishing brutality, often affecting civilian populations on a massive scale, leading to the death of innocent victims, mass displacement, social destabilization and long-lasting wounds,” Pope Leo wrote. Following the dual World Wars, he observed, many nations and international bodies of governance and regulation laid heavy restrictions on the use of military force in an effort to avoid perpetuating wars and risking plunging the world into a second global conflict. “Even during the Cold War,” the pope noted, “despite the existence of serious conflicts, there remained the awareness that a new world war had to be avoided at all costs.”
As in so many instances in ages past, the world would do well to heed the voice of the Shepherd of Rome.
“Today, however, we are witnessing a real paradigm shift in public discourse and in decisions regarding rearmament, with a troubling revival of war as an instrument of international politics, while the very ethical principles that had previously limited its use are being eroded,” Pope Leo continued. He warned that “polarizing media narratives” also play a role in normalizing international conflict and violence, in conjunction with “a selective or distorted rewriting of the past” due to the fact that many of those who experienced historical horrors, like World War II, have passed away. “Without a living memory of the horrors of war, political decisions risk being made on the basis of power alone, without any consideration for the long-term consequences.”
“It is in this context that humanity is slipping into a violent culture of power, where peace no longer appears as a responsibility to be taken on, but as a fragile interval between conflicts,” Pope Leo asserted. “Today, more than ever, without prejudice to the right to self-defense in the strictest sense, it is important to reaffirm that the ‘just war’ theory, which has all too often been used to justify any kind of war, is now outdated.”
The Catholic theory of just war has been developed over centuries, rooted in Sacred Scripture and the recognition of war as an evil to be avoided at nearly any cost. Doctor of the Church St. Augustine of Hippo laid the foundation for the modern understanding of the just war theory, noting that violence for the sake of vengeance is sinful, but that war conducted by the lawful authorities for the sake of the common good may be permissible. “We do not seek peace in order to be at war, but we go to war that we may have peace. Be peaceful, therefore, in warring,” Augustine wrote in a letter to the Roman General Boniface. Like Pope Leo, however, Augustine detested that even just wars were a necessity. “The wise man will wage just wars,” he wrote, “but he will all the rather lament the necessity of just wars.”
Another Doctor of the Church, the ingenious Dominican theologian and philosopher St. Thomas Aquinas, systematized the just war theory (jus ad bellum) in his 13th century Summa Theologica. According to Aquinas, a war can only be just if it meets three requirements. First, the war must be declared by a legitimate, lawful authority; it cannot be initiated or carried out by a private individual. Second, there must be a just cause. “Those who are attacked should be attacked because they deserve it on account of some fault,” Aquinas wrote, relying on Augustine’s arguments. “A just war is wont to be described as one that avenges wrongs, when a nation or state has to be punished, for refusing to make amends.” Finally, the war must be waged for the right intentions. “The belligerents should have a rightful intention, so that they intend the advancement of good, or the avoidance of evil,” Aquinas stipulated. “True religion looks upon as peaceful those wars that are waged not for motives of aggrandizement or cruelty, but with the object of securing peace, of punishing evil-doers, and of uplifting the good.”
Bear in mind that Aquinas was writing at a time when wars primarily concerned Christian kingdoms fighting against one another. The repelling of Islamic attempts at European conquest were understood almost innately as justified in self-defense — in defense of one’s people, in defense of one’s land, and in defense of the Christian faith. It was when Christian kings in Europe were declaring war against one another, as they frequently did, that the Church had to provide moral guidance, clarity, and regulation.
In the 16th century, another Dominican theologian, Francisco de Vitoria, would adapt the stipulations of Augustine and Aquinas for a more modern age. De Vitoria expanded on the system defined by Aquinas, clarifying that particular causes for war — such as expansion of one’s empire or kingdom, the gaining of glory, or the imposition of the Cristian faith by means of conquest and violence — were not just causes and would automatically invalidate the moral permissibility of a war. In the end, de Vitoria concluded that the only real just cause for a war was an injury received. Defensive wars, he reasoned, were always justified, while offensive wars were only justified in order to right a wrong or recover something that had been wrongly taken. Furthermore, de Vitoria stipulated the rule of proportionality: the damage done by a war cannot be greater than the damage done by the wrong which the war seeks to right. He also introduced the concept that war ought to be a last resort and can only be morally permissible once all avenues of diplomacy and negotiation have been exhausted.
Som years later, Jesuit Francisco Suárez would author Disputatio de bello, one of the most influential treatises on the development of just war theory. Recognizing that wars must be governed by the virtue of justice, Suárez also argued that they ought to be balanced by the virtue of charity. Like Augustine, Aquinas, and de Vitoria, Suárez stipulated that a war can only be just if it is declared by a legitimate authority, waged on behalf of a just cause, waged for the right intentions, and must maintain proportionality to the wrong being redressed. Like de Vitoria, the Jesuit theologian also suggested that war should be a “last resort,” but went further and stipulated that a war is unjust if there is some alternative means of redressing the wrong in question. Suárez also stipulated that there must be a reasonable hope of success for a war to be just and urged rulers to take into account charity when waging war. For example, a wealthy nation with a powerful military would be uncharitable in waging a devastating war against a smaller, poorer, weaker nation over a lesser wrong or injury.
The Catechism of the Catholic Church synthesizes and summarizes the centuries of just war theory, stipulating that war is generally to be avoided. “All citizens and all governments are obliged to work for the avoidance of war. However, ‘as long as the danger of war persists and there is no international authority with the necessary competence and power, governments cannot be denied the right of lawful self-defense, once all peace efforts have failed,’” the Catechism states, citing the Second Vatican Council’s Gaudium et Spes. The Catechism further orders that all conditions for a just war must be met in order for the war to be considered just:
[T]he damage inflicted by the aggressor on the nation or community of nations must be lasting, grave, and certain; all other means of putting an end to it must have been shown to be impractical or ineffective; there must be serious prospects of success; the use of arms must not produce evils and disorders graver than the evil to be eliminated. The power of modem means of destruction weighs very heavily in evaluating this condition.
In his recent encyclical, Pope Leo is not repudiating the centuries of development of the just war theory. He is, rather, continuing its development. When Augustine and Aquinas were writing, warfare consisted of men in chain mail and armor carrying bladed weapons, facing off on a field of battle. The most destructive of weapons at the time were catapults and trebuchets, flinging boulders or flaming projectiles into cities of timber and masonry. By the time that de Vitoria and Suárez were addressing the issue of war, swords were augmented, and soon replaced by muskets, which were in turn replaced by automatic rifles, which paled in comparison to the treat of mortars and bombs. Recent years have seen the advent of various forms of missiles and, now, the use of A.I. in launching, steering, and detonating those missiles, while the ghastly specter of nuclear weapons looms over every battlefield.
Pope Leo is not, certainly, suggesting that no war can now be a just war. In fact, he explicitly rejects such an argument and designates wars of self-defense as nearly always justified. The pontiff is, however, warning that the rapid advance in weapons technology (if it can be truly called an advance), the disturbing integration of A.I. into warfare, and the seeming rise of a casual approach to war are poised to form an unholy trinity which may very well, if not restrained, resurrect from its shallow grave the horror of a world war.
The last time that the world went to war, the U.S. and Germany were racing to develop what would become the nuclear bomb. Today, at least nine nations possess a cumulative total of more than 12,000 nuclear warheads. While the Second World War ended when the U.S. deployed two of those nuclear bombs, there is no guarantee that a third World War, drawn along lines of economy and trade, religious and political alliances, would leave any survivors. As in so many instances in ages past, the world would do well to heed the voice of the Shepherd of Rome.
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