Just war theory is one of the great moral and intellectual achievements of the Church, standing on the pillars of arguably her two greatest thinkers—St. Augustine and St. Thomas Aquinas. Outside of Catholicism, it is without peer, which is why it is studied so extensively, even in secular contexts.

Today, just war theory has perhaps become more urgent than it was perhaps even 40 years ago. With the Cold War long gone and the war on terror still very much with us, understanding what constitutes a “just” war in the eyes of the Church is as important as it has ever been.

The need for a deeper look at what the Church says about war is underscored by the nature of modern warfare: often the United States finds itself fighting against terrorist organizations and militias, in which clearly defining the enemy and pursuing him can entail all sorts of moral hazards. From President George Bush’s preemptive war against Iraq to President Obama’s drone assassinations, the rules seem to have changed—or, at least, our government is trying to change them.

So, what does the Church say about just war?

Traditionally, just war theory is based on principles taken from Aquinas’ Summa Theologica. Aquinas says that three things are necessary in order for a war to be just:

1. Legitimate authority: First, the war must be declared by a legitimate authority. “For it is not the business of a private individual to declare war, because he can seek redress of his rights from the tribunal of his superior,” Aquinas says. “Moreover it is not the business of a private individual to summon together the people, which has to be done in wartime. And as the care of the common weal is committed to those who are in authority, it is their business to watch over the common wealth of the city, kingdom, or province subject to them.”

2. Just cause: Secondly, there must be a “just cause”—or legitimate reason for the war. Quoting Augustine, Aquinas says those reasons could include avenging a wrong or restoring something that has been “seized unjustly.”

3. A rightful intention: Those waging the war must have good intentions. You can’t start justified war with malicious intent. The war must serve “the advancement of good, or the avoidance of evil,” Aquinas says.

Since the Summa, the Church has developed Aquinas’ thought further, adding three more criteria. According to the Catholic Education Resource Center they are:

4. Last resort: The war must be a last resort. As the Catechism of the Catholic Church states, “all other means of putting an end to it must have been shown to be impractical or ineffective.”

5. Proportionality: The war must be a proportional response to the evil it aims to address. As the center puts it, “What good is it to wage war if it leaves the country in total devastation with no one really being the winner?”

6. Probability of success: There has to be a good chance of winning the war—or, in the words of the catechism, “serious prospects of success.”

Although these are not typically enumerated as separate points, the catechism sets out two other key stipulations. First, it describes just wars as wars of defense. This rules out wars of conquest and makes preemptive wars highly questionable, at best. Second, the catechism says that the acts of the aggressor against which nations are defending themselves must be “lasting, grave, and certain.”

In the twentieth century in particular, the Church has fleshed out just war theory in one important area. Not only is it important that the cause of the war be just, but also, the way the war is waged must be just as well. The overriding concern here is the deliberate targeting of citizens or callously neglecting their safety. It’s little surprise that the Second Vatican Council addressed this concern in the wake of World War II, in which 40 million civilians perished—more than double the number of soldiers.

Gaudium et Spes, also known as The Pastoral Constitution on the Church in the Modern World, notes that, “For acts of war involving these weapons can inflict massive and indiscriminate destruction, thus going far beyond the bounds of legitimate defense.” In the same section it went on to declare that, “Any act of war aimed indiscriminately at the destruction of entire cities or extensive areas along with their population is a crime against God and man himself. It merits unequivocal and unhesitating condemnation.”

The council fathers needed to look no further than the horrors of last world war, in which both sides were guilty of targeting civilians. The Nazis used “buzz bombs” to demolish whole blocks in London while the Allies fire bombed Hamburg and Dresden, resulting in the deaths of nearly 200,000 citizens combined, according to the Catholic Education Resource Center. Then, of course, there was the dropping of the atomic bombs on the Japanese cities of Nagasaki and Hiroshima.

So, what are the implications of all this for the current crisis for Iran? Is a war with Iran justified right now?

To start with, it’s worth noting that the crisis reportedly stems from Iranian aggression—state-sponsored terrorism, to which the United States responded with the drone assassination of Iranian General Qasem Soleimani. So the bare minimum criteria of the war being defensive and aiming to address a wrong appear to be potentially met.

But it’s the last three criteria seem particularly salient at the moment. Is the war a last resort? As of this writing, the answer is no. As has been well-documented in the news, much can be done to avoid war. The first step is to de-escalate the crisis. Responding to attacks with counterattacks is a sure path to war.

Second, we must consider whether war would be a proportional response. Will the good it brings about outweigh the evils it causes? It’s hard to see how it would. We have to look no further than what’s happened in neighboring Iraq over the last 15 years to see how devastating a war of occupation can be on a Middle Eastern country, as well as those occupying it.

Third, the probability of success is questionable. Don’t sneer at Iran. According to the BBC, Iran has 523,000 active military personnel. The military also has access to modern technology like jet fighters, submarines, and missiles. One defense expert, Robert Farley, warns that war with Iran would be even worse than the Iraq invasion, with “little prospect for success.”

“Regime change might work, but there’s little good reason to believe the chances of such are high,” Farley concludes. “A war would incur serious costs on Iran, but would also commit the United States to the destruction of the Islamic Republic, a process that could take decades, if it succeeds at all.”

Thus, while an Iranian war might seem initially justified, the difficulties such a war would entail make it unlikely to satisfy all the Church’s standards. So let’s hope the discussion remains purely a ‘theoretical’ one.