By Robert Royal.

Nuclear weapons, like other modern technological developments, have placed great strains on traditional moral principles. Just as modern medicine has changed our appreciation of the beginning and end of human life, the tremendous destructive power of modern weapons, nuclear and not, has made careful thought about war not only urgent, but – to use the fashionable term – existential.

That's probably the main reason why the Vatican has seemed quasi-pacifist in recent decades. But the Church has a well-developed set of criteria about just and unjust uses of force. Indeed, in the past, it even – rightly – called for crusades. (I'll explain another time.) But those criteria – still valid in themselves – need further elaboration to confront the conditions in which we find ourselves.

I have immediate family members who fought in Iraq and Afghanistan, been active in U.S. diplomacy in the Middle East, and worked in the Pentagon managing defense preparedness. Some of my grandchildren have been forced into air-raid shelters in Jerusalem; the others may someday face terrorism at home or, themselves, have to take part in foreign wars. Millions of Americans – and not only Americans – have similar stories. And unless we keep the human costs of warfare front and center in our minds, we may be tempted to take just-war theory as merely a political or intellectual exercise.

That said, there are, of course, things worth dying for – and, regrettably, things worth killing for. That's precisely why just-war theory, a tradition of moral reflection that began in the ancient world, was developed – notably by Augustine and Aquinas, and is the common heritage of most modern militaries. Some of the best-informed students I've ever had on just wat over the years learned that tradition during U. S. military training. Academic types often scoff at this, but it's true.

A good summary of just-war principles can be found here. (Our friend Phil Lawler has been re-examining them in strict fidelity to the Catholic tradition online here). But I want to look closely at just a few of them here to highlight some special circumstances that they now face.

I'm not sure whether the U. S. attack on Iran these past few days is justified. A lot of people already claim to know, one way or the other. But I've seen enough similar situations to be willing to suspend judgment until we know more. (I've misjudged in the past.) Still, I am sure that the way to decide should be on Catholic just-war grounds, not just the wearying and utterly predictable pro- and anti-Trump tug-of-war.

The first criterion is last resort. Resort to arms is a life-and-death matter. It should only be done when other means of addressing a threat have failed. But who decides when all reasonable alternatives have been exhausted? You can always claim that something else might be pursued. In the meantime, great evils may spread:

Nature's polluted,

There's man in every secret corner of her

Doing damned, wicked deeds.

The answer is that a legitimate authority has the responsibility to decide. But also must explain how everything reasonable has been tried, what the threat is, and why it's necessary, right now, to meet it.

The president hasn't said nearly enough about this. There are rumors that Iran was planning a strike against U.S. forces. If so, we need an authoritative statement about that – and more.

Hamas, Hezbollah, or the Houthis (and maybe a few campus fellow-travelers) may lose sleep over the fall of the Islamic Republic. No one else will. Everybody has agreed that "Iran must not develop a nuclear weapon" (existential threat), but done little beyond talk – for half a century. So it's good that the president has put the attack in terms of defense, both immediate and long-term. But we still need to know much more.

A second criterion is just cause: Wars of conquest, in our tradition, are never just. Our intention must be to achieve some good by righting a wrong, actual or imminent. We c...