It was later developed systematically by St Thomas Aquinas, who anchored it within the broader framework of natural law and moral philosophy. The aim was not to glorify war, but to establish clear moral boundaries within which the use of armed force could be legitimate and to limit it strictly to cases of necessary self-defense or the protection of the innocent.

The current Catechism of the Catholic Church, in paragraph 2309, sets out four basic conditions that must all be met: the damage inflicted by the aggressor on a nation or community of nations must be lasting, grave and certain; all other means of putting an end to it must have proved impractical or ineffective; there must be serious prospects of success; and the use of arms must not produce evils and disorders graver than the evil to be eliminated.

These criteria set a high bar. They do not provide a free license to wage war, but establish an exceptional instrument of self-defense or protection of the innocent that must pass a strict moral test. It is precisely this rigor that has historically served as a check on the arbitrary use of violence and as a moral compass for rulers.

The Stretching of Self-Defense

In recent decades, however, the theory of just war has often been turned into a flexible political tool in international affairs. States use it to legitimize actions that would be difficult to justify under a strict reading of the criteria. Rather than being confined to exceptional cases of self-defense, the concept has become a broad cover for preventive operations, geopolitical interventions and the pursuit of national interests.

One of the most striking examples was the 2003 invasion of Iraq. The US-led coalition cited an imminent threat from weapons of mass destruction and alleged links between Saddam Hussein’s regime and terrorist networks. It later became clear that these justifications were, at best, questionable and in some cases unfounded. Preventive war thus effectively replaced classical self-defense under international law. The operation, which was intended to be a swift intervention, resulted in the long-term destabilization of the region, the rise of ISIS and hundreds of thousands of casualties.

A similar mechanism was at work in Libya in 2011. United Nations Security Council Resolution 1973 authorized action to protect civilians from Muammar Gaddafi’s regime. The result, however, was the destabilization of the country, the collapse of state structures, civil war and long-term regional instability, followed by waves of migration to Europe. The fourth condition of the catechism – that an intervention must not cause greater harm than the original evil – was clearly violated.

A similar debate is now unfolding over conflicts in the Middle East. In recent years, Israel has justified military operations against Hezbollah in Lebanon and against Iranian proxy forces as necessary self-defense against an existential threat. While many analysts recognize Israel’s right to defend itself against rocket fire and terrorist attacks, critics point to the scale of the operations and the number of civilian casualties, raising questions about whether they still meet the criteria of proportionality and distinction between combatants and civilians.

Current tensions surrounding Iran create further complications. Pre-emptive strikes against Iranian nuclear facilities or Revolutionary Guard commanders are often presented as necessary self-defense against a future threat. From the perspective of just war theory, however, serious questions arise: Is this a genuine and imminent threat, or a pre-emptive action based on presumed future aggression? And can such an operation meet the requirement that it must not cause more harm than the danger it seeks to avert?

The war in Ukraine also deserves particular attention. Russia’s invasion is a clear case of aggression aimed at expanding influence and redrawing borders by force. Prior geopolitical tensions, including NATO expansion and internal Ukrainian divisions, may form part of the wider background to the conflict, but they do not alter the basic moral and legal fact of Russian aggression.

Other conflicts follow the same pattern. Great powers often label their actions as “self-defense” even when they involve pre-emptive strikes or the pursuit of geopolitical interests. Hybrid forms of conflict further complicate the application of traditional criteria. In the age of drones and artificial intelligence, the line between war and peace is becoming blurred, giving politicians broad leeway for interpretation.

The repeated stretching of just war reasoning is gradually eroding moral standards in international relations.

Theological Context

In Magnifica Humanitas, Pope Leo XIV does not deny the right to self-defense in the strictest sense. On the contrary, he explicitly affirms it. His criticism is directed at the fact that the theory of just war has become too flexible and too easily abused. In doing so, he refers to earlier documents, including Pope Francis’s encyclical Fratelli Tutti, which warned of the dangers of an overly broad interpretation.

Theologian Edward Feser and other commentators emphasize that such reservations are not new. As early as the mid-20th century, Cardinal Alfredo Ottaviani warned that modern weapons of mass destruction would hardly meet the conditions for a just war. Leo XIV is therefore continuing a tradition of reflection within the Church rather than marking a radical break.

At the same time, the encyclical highlights the new challenges of the 21st century. Artificial intelligence in weapons systems reduces personal responsibility, accelerates decision-making and makes it harder to distinguish between combatants and civilians.

Hybrid threats, from cyberattacks to targeted disinformation, require an updating of moral criteria, not their abolition.

When Doctrine Becomes a Pretext

When the concept of a just war becomes a broadly interpretable tool of international politics, its original meaning is gradually eroded. According to Phil Lawler, this leads to a loss of the ability to distinguish clearly between legitimate defense and other forms of armed conflict.

In such cases, international law and public debate tend to conform more closely to the current interests of great powers than to universal ethical criteria.

According to several analyses, this development has long-term consequences. If the parties involved can present almost every armed conflict as “just”, the moral authority of the doctrine declines and the credibility of the international legal framework is weakened. Historical experience from the past two decades shows that an overly broad interpretation can lead to the normalization of preventive action and the erosion of diplomatic solutions.

In this context, Pope Leo XIV’s encyclical Magnifica Humanitas does not primarily object to the theory of just war itself, but draws attention to its frequent abuse in practice. The document emphasizes the need for stricter application of the criteria, greater emphasis on diplomatic instruments and careful consideration of the new technologies that are changing the nature of modern conflict.

Whether, under current conditions, the theory needs to be radically revised, or whether its traditional principles should instead be applied more rigorously and consistently, remains a matter of scholarly debate among theologians and ethicists. For many, any such update should aim at greater caution and a stronger role for preventive diplomacy, so that the doctrine can once again fulfill its original purpose: to limit war, not to justify it.