Since the beginning of the Iran war, Pope Leo XIV has frequently called for peace, cautioning that the “” makes military force seem preferable to diplomacy. Although U.S. Vice President JD Vance, a Catholic, , a growing choir of Catholic voices has criticized the conflict by invoking ” – 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 “.” A month later, the prelate leading the U.S. military’s Catholic chaplaincy delivered a stark assessment: The war was . The raised similar concerns.
Many faiths have teachings about when war is or is not considered justified, , . In the Christian just war tradition, battle is never holy – “,” in Leo’s words – but it is sometimes considered necessary.
That tradition traces its roots to the fifth-century theologian . A millennium later, systematized the church’s , 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 holds this authority, since the 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 , and Congress controls military funding. Yet the Constitution simultaneously grants the president broad authority to .
The attempted to balance these principles by requiring presidents to seek congressional authorization for any use of force .
2. Just cause
Traditionally, Christian theologians argued that could justify war.
Some causes can never be just. For example, the 16th-century scholar explicitly ruled out “” as legitimate causes for war.
The Trump administration has offered rationales for the Iran war – even humanitarian ones, telling Iranians suffering under that “” – 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 “.” International law and the just war tradition uphold states’ right to self-defense.
But the law permits force only when . And Secretary of State Marco Rubio said , 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 .
In addition to self-defense, Trump claimed a need to prevent future threats – otherwise called – such as or that could reach the United States.
Iran has a history of , which it claims is for civilian use. Experts for the country to produce a nuclear weapon. In 2025, the International Atomic Energy Agency declared Iran on nuclear nonproliferation. However, international law .
Trump has also stated that the war would ensure Iran . The regime .
This is a , 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 “.” Augustine saw as evil intents. “The common good of the commonwealth” should motivate the decision to go to war, , 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. , for example, cast doubt on the Trump administration’s humanitarian claims.
In March, the president told the Financial Times that “.” In , he wrote, “With a little more time, we can easily OPEN THE HORMUZ STRAIT, TAKE THE OIL, & MAKE A FORTUNE.” , however, would violate right intent.
4. Proportionality
War . But today’s , a summary of the church’s teachings, states that the “use of arms 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 . An estimated Iranians have . have been destroyed.
The disruption of oil production and trade translates into higher energy and fertilizer prices, which – hitting the world’s .
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 “.”
Arguably, U.S. officials did not give diplomacy sufficient time to work. Days before the war began, some analysts believed that a . 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 “.” The Guardian reported that the U.K.’s national security adviser, who was also present at those February talks, .
Experts suggest that the U.S. negotiating team’s and the 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 “,’” as international relations scholar Frances V. Harbour put it. Limited goals are more likely to succeed than expansive ones.
The war has . But the knowledge needed to build them remains, and without a diplomatic solution, Iran is to develop such technologies.
Similarly, force can disrupt Iran’s proxy networks and raise the costs of maintaining them, but 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 .
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