Since the beginning of the Iran war, Pope Leo XIV has frequently called for peace, cautioning that the "delusion of omnipotence" makes military force seem preferable to diplomacy. Although U.S. Vice President JD Vance, a Catholic, criticized some of the pope's comments, a growing choir of Catholic voices has criticized the conflict by invoking the concept of "just war" – an evolving tradition that has guided Christian thinking about war and peace for 1,500 years.

In March, the archbishop of Washington said the war failed "to meet the just war threshold." A month later, the prelate leading the U.S. military's Catholic chaplaincy delivered a stark assessment: The war was not justified. The Vatican's secretary of state raised similar concerns.

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Many faiths have teachings about when war is or is not considered justified, including Judaism, Islam and Hinduism. In the Christian just war tradition, battle is never holy – "God does not bless any conflict," in Leo's words – but it is sometimes considered necessary.

That tradition traces its roots to the fifth-century theologian St. Augustine. A millennium later, St. Thomas Aquinas systematized the church's just war teachings, establishing three basic criteria to assess the justifiable use of force: authority, cause and intention. Over time, three more principles emerged: proportionality, last resort and likelihood of success.

Here's how they could apply today:

1. Legitimate authority

Historically, the conversation about a war's justness began by asking whether a responsible sovereign had declared it.

Today, some just war scholars argue only the United Nations holds this authority, since the U.N. charter forbids the use of force against another nation except for self-defense.

In the United States, the boundary between presidential and congressional authority for war is contested. According to the U.S. Constitution, only Congress can declare war, and Congress controls military funding. Yet the Constitution simultaneously grants the president broad authority to command military operations.

The 1973 War Powers Resolution attempted to balance these principles by requiring presidents to seek congressional authorization for any use of force lasting more than 60 days.

2. Just cause

Traditionally, Christian theologians argued that self-defense and righting wrongs could justify war.

Some causes can never be just. For example, the 16th-century scholar Francisco de Vitoria explicitly ruled out "difference of religion" and "enlargement of the empire" as legitimate causes for war.

The Trump administration has offered numerous and shifting rationales for the Iran war – even humanitarian ones, telling Iranians suffering under a brutally oppressive regime that "the hour of your freedom is at hand" – which makes it difficult to assess the justness of its cause.

One of the main explanations U.S. officials have offered, for example, is self-defense. On the war's first day, Trump declared the objective was to eliminate "imminent threats from the Iranian regime." International law and the just war tradition uphold states' right to self-defense.

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But the law permits force only when necessary to end an ongoing attack or to avert an imminent one. And Secretary of State Marco Rubio said the U.S. attacked because of a planned Israeli strike, casting doubt on the idea of imminent threat: "We knew that if we didn't preemptively go after (Iran) before (Israel) launched those attacks, we would suffer higher casualties." Pentagon briefers also told Congress the Iranian threat was not imminent.

In addition to self-defense, Trump claimed a need to prevent future threats – otherwise called preventive war – such as nuclear weapons or longer-range missiles that could reach the United States.

Iran has a history of covert nuclear research, which it claims is for civilian use. Experts debate how long it would take for the country to produce a nuclear weapon. In 2025, the International Atomic Energy Agency declared Iran was not complying with agreements on nuclear nonproliferation. However, international law prohibits preventive war.

Trump has also stated that the war would ensure Iran cannot support "terrorist proxies" abroad. The regime funds and equips Hamas and the Lebanese militant group Hezbollah.

This is a gray area of international law, but providing financial and material aid alone is generally not considered sufficient justification for an attack.

3. Right intent

Just cause alone is insufficient to render a war just.

Aquinas cautioned that even a war declared by a "legitimate authority, and for a just cause" could "be rendered unlawful through a wicked intention." Augustine saw love of violence, cruelty or power as evil intents. "The common good of the commonwealth" should motivate the decision to go to war, wrote Vitoria, the 16th-century theologian – not the leader's personal gain or honor.

Assessing right intent is hard, but a government's conduct and rhetoric can offer clues. Attacks on civilian infrastructure, for example, cast doubt on the Trump administration's humanitarian claims.

READ MORE: A president and a pope: Two of the world's most influential Americans at odds over Iran

In March, the president told the Financial Times that "my favorite thing is to take the oil in Iran." In an April post on Truth Social, he wrote, "With a little more time, we can easily OPEN THE HORMUZ STRAIT, TAKE THE OIL, & MAKE A ⁠FORTUNE." Pursuing economic interests, however, would violate right intent.

4. Proportionality

War is always destructive. But today's Catholic Catechism, a summary of the church's teachings, states that the "use of arms must not produce evils and disorders graver than the evil to be eliminated." In other words, the just war tradition holds that a war is justified only if the harm it causes is proportional to the good it seeks to achieve.

As of April 7, 2026, more than 1,600 Iranian civilians have been killed, including more than 200 children. An estimated 3 million Iranians have been displaced. Schools and health care facilities have been destroyed.

The disruption of oil production and trade translates into higher energy and fertilizer prices, which raise food prices – hitting the world's poorest the hardest.

Whether the costs of the Iran war are proportionate depends on which of the administration's stated aims one believes.

5. Last resort

The Catholic Catechism declares war can be legitimate only if "all other means" of putting an end to an aggressor's harms "have been shown to be impractical or ineffective."

READ MORE: War is reducing international law to 'ashes,' Pope Leo laments in Ash Wednesday liturgy

Arguably, U.S. officials did not give diplomacy sufficient time to work. Days before the war began, some analysts believed that a deal was close. Oman's foreign minister, who hosted negotiations in February, said "it was a shock but not a surprise" that the U.S. and Israel attacked, after peace "had briefly appeared really possible." The Guardian reported that the U.K.'s national security adviser, who was also present at those February talks, expressed similar sentiments.

Experts suggest that the U.S. negotiating team's lack of technical expertise and the tight timeline contributed to failure.

6. Likelihood of success

To be justified, the use of force must be likely to accomplish the war's aims. Ethicists debate the exact bar but agree that success must be "more likely than a mere 'hope,' 'chance,' or 'possibility,'" as international relations scholar Frances V. Harbour put it. Limited goals are more likely to succeed than expansive ones.

The war has degraded Iran's nuclear and missile programs. But the knowledge needed to build them remains, and without a diplomatic solution, Iran is likely to continue its efforts to develop such technologies.

Similarly, force can disrupt Iran's proxy networks and raise the costs of maintaining them, but regional diplomacy and cooperation have a better chance of resolving such long-running concerns.

Ultimately, I believe a lack of clarity about the war's goals diminishes the likelihood of success. Wars require more than military victories; there must be a coherent plan for ending the fighting such that a "better peace" is established.

This article is republished from The Conversation under a Creative Commons license. Read the original article.