Beyond Just-War Theory
By Richard Finn, OP
Pope Leo XIV’s first encyclical, Magnifica Humanitas, offers an inspiring vision of what it means for us to be Catholics. It demands our social engagement, our commitment to building a more just, sustainable, and compassionate world, by rooting such action in the fundamentals of our Christian faith. The Pope is at pains to show how the Church’s Social Doctrine (more often referred to elsewhere as ‘Catholic Social Teaching’) emerges seamlessly from Church doctrine on our creation and redemption. In Christ, we are drawn into the love shared among the persons of the Trinity and, so, inescapably, called into sharing God’s love for each and every person, who is made in His image and likeness and so endowed with an inalienable dignity to be revered and respected. Magnifica Humanitas then reviews the tradition of papal encyclicals on social justice that has followed the publication of Rerum Novarum in 1891 to identify moral principles of enduring relevance in addressing the many urgent issues raised in the present day by Artificial Intelligence. Pope Leo doesn’t offer all the answers – far from it! Instead, he outlines the angle of approach, moral perspectives to be adopted and brought to bear on difficult questions in honest, constructive, dialogue.
One key area with which Magnifica Humanitas requires us to wrestle is the untold damage wrought by an all-too ready recourse to war as ‘an instrument of international politics’ (Paragraph 190); and the yet greater harms that may occur through the changing nature of modern warfare as its weaponry is revolutionised by AI. Pope Leo starkly warns: ‘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important to reaffirm that the “just war” theory, which has all too often been used to justify any kind of war, is now outdated’ (Paragraph 192). That judgment may come shock doughty champions of an ethical tradition that counts two Doctors of the Church, St Augustine of Hippo and St Thomas Aquinas, amongst its chief proponents. So, what sense can we make of Pope Leo’s verdict?
It helps first to remember the different elements that comprise so-called ‘Just War Theory’. There are the conditions that must be met if there is to be just cause for fighting at all, and conditions which limit how warfare may be conducted. Only the credible threat or infliction of grave injustices may justify recourse to arms to prevent or redress those injustices, and such recourse must be the last resort after other avenues for resolving the matter have been exhausted. There must be good grounds for holding that a war fought with due observance of ethical imperatives will succeed in securing the good it aims at without incurring even greater evils. Those who then prosecute the war must have due authority so to act.
Within war, ‘Just War Theory’ holds that it is never right to target non-combatants, those not involved (directly or indirectly) in waging war, nor to destroy the vital infrastructure that maintains civilian life. The force used must be proportionate to the size and strength of the legitimate targets identified, so that collateral damage, and in particular civilian casualties, are minimised.
There is no reason to think Pope Leo rejects these conditions that are, theoretically, meant to limit the grave harms done by war. Indeed, he laments the erosion of such ethical principles. The problem, rather, lies in the difficulty of applying the conditions successfully. That is to say, the difficulty lies in making good (rational) use of them, either in the run-up to a likely war or in its prosecution once underway. The ‘theory’ doesn’t, in practice, deliver the protections it is meant to ensure or, if not ensure, at least promote.
The problems are multiple. In part, the difficulty lies with the principal ‘actors’, those with the with the command and who claim authority to launch or direct a war. Some are simply not guided by the ethical principles or goals envisaged by Just War Theory, and their control over the armed forces brooks no dissent from their orders. Be they Heads of State or leaders of armed movements, ‘Just War Theory’ has little or no purchase on their decisions. Where Heads of State are constitutionally required to gain authorization for war from those who do respect the conditions laid down by ‘Just War Theory’, Pope Leo has drawn attention in particular to how a ‘culture of power’ fuels a ‘false realism that insists that there is no alternative’ to war (Paragraph 188). The widespread diffusion of misinformation, the distorted and stunted nature of political debate, deprives people of the knowledge needed successfully to apply the criteria of ‘Just War Theory’ and so restrain warmongers. While the theory respects conscientious objection on the part of civilians and the refusal by soldiers to carry out war crimes, the actual training and execution of warfare, including the use of child soldiers by warlords, makes the latter refusal all too rare.
On the world stage, political leaders collaborated in the aftermath of World War Two in order to reduce the threat of renewed destruction through war. They subordinated national sovereignty in some measure to international law which limited the authority of States to act unilaterally in going to war, and which further defined war crimes against humanity, together with a crime of genocide. They established the United Nations and international courts to uphold these laws. Yet, the legislation lacks sufficient teeth, and the ability of the United Nations to restrain some of the most powerful actors is hobbled by the nature and operation of the so-called Security Council. ‘Just War Theory’ needs a tougher international legal apparatus to gain adequate purchase.
A further difficulty in putting ‘Just War Theory’ to good use lies in the blurring of traditional lines that facilitated its application. The line between war and peace is blurred through the language of ‘military operations’ with consequences for how, or by what body, such fighting is authorised within the State when there is no formal declaration of war. Worse, warfare is no longer confined to a front line or its immediate hinterland, a battlefield distant or at least separate from civilian life, but is conducted at high speed against numerous targets, including command posts, dispersed within towns and cities. In this context, where civilians have no time and little opportunity to flee, civilian casualties multiply, and it becomes harder to determine the proportionality of any single attack. Cyberwarfare further blurs the line between military and civilian targets as it seeks to disrupt systems vital to all sectors of society.
Meanwhile, drone warfare and the use of AI increase the degree to which modern warfare distances protagonists from appreciating the flesh and blood reality of the men and women under attack. Serious consideration is currently being given to the enabling of drones to select and attack targets without reference to, or authority from, a remote human operator who is required to make a moral judgment of whether or not to attack a given individual, group, or installation. Pope Leo is adamant on this point that ‘it is not permissible to entrust lethal or otherwise irreversible decisions to artificial systems’. Those ‘who design, train, authorize and employ’ this technology ‘must be held accountable for their decisions’, and its operation must admit of a ‘moral timeframe’ within which the handler can identify and protect civilians (Paragraphs 198-199).
What, then, happens next? It may not be a matter of replacing ‘Just War Theory’ but of recovering respect for law, including international law, of strengthening the hand and reach of international courts, and of building a culture of peace-building through diplomacy. It means better access to, and interpretation of intelligence. Pope Leo XIV in Magnifica Humanitas doesn’t provide a blueprint but sets an agenda for debate.
Reproduced with kind permission from The Dominicans, Summer 2026.
