The right of defense, and the duty of peace.

Summary

In his encyclical Magnifica Humanitas of May 15, 2026, Pope Leo XIV wrote that, “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important to reaffirm that the” just war theory, “which has all too often been used to justify any kind of war, is now outdated”⁠.¹ His note cites Pope Francis in Fratelli Tutti, who recalled the Catechism’s “rigorous conditions of moral legitimacy” for defense and warned against “an overly broad interpretation of this potential right”⁠.^1,2

The Catechism still teaches that “governments cannot be denied the right of lawful self-defense, once all peace efforts have failed”⁠,³ under strict conditions that are “the traditional elements enumerated in what is called the” just war doctrine⁠.³ It also teaches that “All citizens and all governments are obliged to work for the avoidance of war”⁠.³

The take: the right of defense remains, under strict conditions; what Pope Leo XIV calls outdated is the just war theory as it “has all too often been used to justify any kind of war”⁠.¹ John XXIII wrote that in the atomic age “it no longer makes sense to maintain that war is a fit instrument with which to repair the violation of justice”⁠.⁴ Pope Francis judged that “it is very difficult nowadays to invoke the rational criteria elaborated in earlier centuries to speak of the possibility of a” just war⁠.² Pope Leo XIV reaffirmed that judgment and pointed to “far more effective and capable tools for promoting human life and resolving conflicts, such as dialogue, diplomacy and forgiveness”⁠.¹ A war of aggression is “intrinsically immoral,” while the leaders of an attacked state “have the right and the duty to organize a defence even using the force of arms”⁠.⁵

The tradition

Augustine taught that a just war is a grief, not a glory: the wise man will “lament the necessity of just wars”⁠,⁶ because “it is the wrongdoing of the opposing party which compels the wise man to wage just wars”⁠.⁶ The real evils of war are not death but “revengeful cruelty, fierce and implacable enmity, wild resistance, and the lust of power”⁠.⁷

Thomas Aquinas asked “Whether it is always sinful to wage war?” and answered with three conditions⁠:⁸

• “First, the authority of the sovereign by whose command the war is to be waged”⁠.⁸
• “Secondly, a just cause is required”⁠.⁸
• “Thirdly, it is necessary that the belligerents should have a rightful intention”⁠,⁸ since a war with authority and cause can still “be rendered unlawful through a wicked intention”⁠.⁸

The Catechism's conditions

The Catechism of the Catholic Church sets out the conditions for legitimate defense by military force. They must all hold “At one and the same time”⁠:³

• “the damage inflicted by the aggressor on the nation or community of nations must be lasting, grave, and certain”⁠;³
• “all other means of putting an end to it must have been shown to be impractical or ineffective”⁠;³
• “there must be serious prospects of success”⁠;³
• “the use of arms must not produce evils and disorders graver than the evil to be eliminated. The power of modern means of destruction weighs very heavily in evaluating this condition”⁠.³

Judging them “belongs to the prudential judgment of those who have responsibility for the common good”⁠.³ Once a war begins, the moral law still binds: “The mere fact that war has regrettably broken out does not mean that everything becomes licit between the warring parties”⁠.³ “Non-combatants, wounded soldiers, and prisoners must be respected and treated humanely,” and “One is morally bound to resist orders that command genocide”⁠.³ “Every act of war directed to the indiscriminate destruction of whole cities or vast areas with their inhabitants is a crime against God and man”⁠.³

Soldiers and conscience

“Those who are sworn to serve their country in the armed forces are servants of the security and freedom of nations”⁠.³ Public authorities may impose “the obligations necessary for national defense”⁠,³ and “should make equitable provision for those who for reasons of conscience refuse to bear arms; these are nonetheless obliged to serve the human community in some other way”⁠.³ “Every member of the armed forces is morally obliged to resist orders that call for perpetrating crimes against the law of nations”⁠.⁵

The Council

“Peace is not merely the absence of war”⁠.⁹ The Council held both lines together. Governments “cannot be denied the right to legitimate defense once every means of peaceful settlement has been exhausted,” but “it is one thing to undertake military action for the just defense of the people, and something else again to seek the subjugation of other nations”⁠.⁹ Scientific weapons “compel us to undertake an evaluation of war with an entirely new attitude”⁠;⁹ “the arms race is an utterly treacherous trap for humanity, and one which ensnares the poor to an intolerable degree”⁠;⁹ and “It is our clear duty, therefore, to strain every muscle in working for the time when all war can be completely outlawed by international consent”⁠.⁹

The popes since 1963

Leo XIV: beyond the just war theory

Magnifica Humanitas places its judgment in a section on “The normalization of war”⁠.¹ It observes “a real paradigm shift in public discourse and in decisions regarding rearmament, with a troubling revival of war as an instrument of international politics”⁠.¹ Against it the encyclical holds that “Humanity possesses far more effective and capable tools for promoting human life and resolving conflicts, such as dialogue, diplomacy and forgiveness”⁠,¹ and it keeps “the principle that armed force should be used only as a last resort in cases of legitimate self-defense”⁠.¹

At Metz on Sept. 28, 2026, the Pope said that the search for peace “must not be confused with a naïve form of pacifism” and that “Promoting peace is an active task,” and in the same address warned against “the illusion that rearmament is the answer”⁠.¹⁷ At Sapienza University on May 14, 2026, he said: “let us not call” defense “a rearmament that increases tensions and insecurity, depletes investment in education and health”⁠.¹⁸

Nuclear weapons

The Compendium of the Church’s social doctrine already asked that “Policies of nuclear deterrence, typical of the Cold War period, must be replaced with concrete measures of disarmament based on dialogue and multilateral negotiations”⁠.⁵ Pope Francis went further: nuclear weapons “create nothing but a false sense of security”⁠,¹⁴ and “a world without nuclear weapons is possible and necessary”⁠.¹⁹ Their “total elimination” is “both a challenge and a moral and humanitarian imperative”⁠.² Pope Leo XIV writes that the idea of deterrence “is based on the irrationality of relations between nations”⁠,²⁰ and he asked the nuclear powers to act, “starting with the countries that possess nuclear arsenals”⁠.¹⁶

Magnifica Humanitas calls the Treaty on the Prohibition of Nuclear Weapons “an important step”⁠.¹ The Holy See’s mission to the United Nations urged states in 2025 “to move beyond the illusory logic of nuclear deterrence”⁠,²¹ and in September 2026 said it “deeply regrets” that the review conference of the Non-Proliferation Treaty “failed to reach a consensus on a substantive outcome”⁠.²²

Weapons and artificial intelligence

The encyclical gives a section to “Weapons and artificial intelligence”⁠.¹ It holds that “it is not permissible to entrust lethal or otherwise irreversible decisions to artificial systems”⁠,¹ and that “No algorithm can make war morally acceptable”⁠.¹ It requires that “the decision to use lethal force cannot be delegated to opaque or automated processes, but must remain under effective, self-aware and responsible human control”⁠.¹

In Antiqua et Nova (2025), the Dicastery for the Doctrine of the Faith and the Dicastery for Culture and Education, quoting Pope Francis, called lethal autonomous weapon systems a “cause for grave ethical concern”⁠.²³ At the U.N. Security Council in May 2025, the Holy See said it “strongly supports the proposal of a legally binding instrument prohibiting lethal autonomous weapons systems (LAWS) by 2026”⁠.²⁴

Defending the innocent

The right of defense carries a duty: “The right to use force for purposes of legitimate defence is associated with the duty to protect and help innocent victims who are not able to defend themselves from acts of aggression”⁠.⁵ “The international community as a whole has the moral obligation to intervene on behalf of those groups whose very survival is threatened,” and “The principle of national sovereignty cannot be claimed as a motive for preventing an intervention in defence of innocent victims”⁠.⁵ Sanctions “must never be used as a means for the direct punishment of an entire population”⁠.⁵ A preventive war “without clear proof that an attack is imminent cannot fail to raise serious moral and juridical questions”⁠.⁵

“Terrorism is to be condemned in the most absolute terms,” its “criminal responsibility is always personal” and “It is a profanation and a blasphemy to declare oneself a terrorist in God’s name”⁠.⁵

The Holy See and today's wars

What Pope Leo XIV and the Holy See have said about the wars of 2025 and 2026, in their own words.

The criteria as the American bishops state them

In 1983 the bishops of the United States set out the tradition as a checklist, beginning from “a strong presumption against war which is binding on all”⁠.⁴⁴ Their list is a teaching of a bishops’ conference, not of the universal Church; the Catechism’s four conditions above are the universal statement.

An unarmed and disarming peace

Pope Leo XIV’s first message for the World Day of Peace roots it in the risen Christ: “The peace of the risen Jesus is unarmed, because his was an unarmed struggle”⁠.²⁰ He names “the disarming path of diplomacy, mediation and international law” and asks for “that disarmament of heart, mind and life”⁠.²⁰ The Catechism had already named where wars begin: “Injustice, excessive economic or social inequalities, envy, distrust, and pride raging among men and nations constantly threaten peace and cause wars”⁠.³