In a letter written around 412 AD, St. Augustine notes that, “If Christian teaching condemned all warfare, then the soldiers in the gospel who were seeking guidance about their security would have been told to throw away their weapons and withdraw entirely from the army.” At the same time, he stresses that warfare is legitimate if and only if it is waged for the sake of peace and in a moral way. Over the centuries, the Catholic Church has matured its teaching on the ethics of warfare. Some may wonder, though, whether that teaching is still valid in a world where the military marshals advanced technology to produce arsenals of immensely destructive weaponry. In this interview, Prof. Gregory Reichberg, a specialist in military ethics, explains his pick of the five best books on the ethics of warfare.

A philosopher by training (with a Ph.D. from Emory University), Gregory Reichberg is a Research Professor at the Peace Research Institute Oslo, where he writes on historical and contemporary issues in military ethics. He is the author of Thomas Aquinas on War and Peace (Cambridge University Press, 2017) and co-editor of several volumes, including The Ethics of War: Classic and Contemporary Readings (Blackwell Publishing, 2006), Religion, War, and Ethics: A Sourcebook of Textual Traditions (Cambridge U P, 2014), and Robotics, AI, and Humanity: Science, Ethics, and Policy (Springer, 2021). His articles have appeared in Catholic journals and magazines, including The Thomist, La Revue Thomiste, Nova & Vetera (English Edition), Proceedings of the American Catholic Philosophical Association, Commonweal, and America Magazine. His current work focuses on artificial intelligence and its implications for military ethics.

  1. Summa Theologiae: Volume 35, Consequences of Charity: 2a2ae. 34-46
    by St. Thomas Aquinas
  2. Vitoria: Political Writings
    edited by Anthony Padgen and Jeremy Lawrence
  3. Freedom in the Modern World
    by Jacques Maritain
  4. The Church of the Word Incarnate
    by Charles Journet
  5. Fratelli tutti
    by Pope Francis

Prof. Reichberg, welcome.
Very good to be here. I start with one qualification. The books that I propose are not necessarily the best books on war and ethics. However, these books provide a window onto important issues relating to war and ethics.

What led you to specialise in military ethics?
My first exposure to military ethics was during the First Gulf War. I was an assistant professor of philosophy at the Catholic University of America in Washington, DC. The then Dean of the School of Philosophy, Jude Dougherty, asked me whether I was willing to be interviewed on the radio about the US-led intervention and whether I could situate this in relation to the thought of Thomas Aquinas. I was paralysed with fear at the prospect of doing that because I had not looked into just war theory at all. So, I found a way to evade the task that he had proposed for me.

But then, several years later, I ended up in Norway and had the opportunity to work at the Peace Research Institute, where I currently work. This was back in 1998. I was hired into a group that was researching humanitarian intervention. It was a significant theme then because of the US-led air war over Kosovo. I was hired into that research group because of my background in mediaeval philosophy, particularly Thomas Aquinas. The roots of humanitarian intervention, and just war more broadly, are often traced back to Thomas Aquinas. The incident at Catholic University put just war on my radar scope, and then I really dove into it as an area of research after coming to Norway in 1998.

Part of the reason for Aquinas's impact was that he took rather complicated discussions about war and ethics, and he reduced theme into three basic principles.


First up, is the section of the Summa theologiae that St. Thomas Aquinas dedicates to war (II-II, q. 40). Presumably, this tops your list because it is a canonical statement of Catholic principles regarding the ethics of war. However, St. Thomas is also drawing on previous theological reflection on the matter, most notably upon St. Augustine. Why have you chosen St. Thomas and what does he add to the preceding reflection?
I could have started with Augustine. There are other important thinkers along the way: for instance, the canon lawyer, Gratian, who took many elements from Augustine's thought on just war and systematised them. What we today call the theory of just war does not emerge directly from Augustine. It emerges remotely from Augustine, but through the prism of the canon lawyer, Gratian. So, I thought it would be good to start with Thomas Aquinas because he was the first thinker to systematise principles around just war. Gratian organised passages from Augustine, but Aquinas distils a core teaching around the idea of just war.

His account in the Summa has had an enormous influence. It is the principal way that the question of rightful engagement in war has been raised within a Catholic framework. However, he is also a point of reference for many secular thinkers or individuals doing research on just war. Part of the reason for Aquinas's impact was that he took rather complicated discussions about war and ethics, and he reduced theme into three basic principles. His treatment of war, the one that's referred to as article 1 of question 40 of the Second Part of the Second Part of the Summa. is about two-and-a-half pages long; maybe even less, depending on the version that you read. Its impact is largely a function of the simplicity of his treatment but also its profoundness. That is why I thought it would be a good starting point.

Earlier I mentioned your monograph, Thomas Aquinas on War and Peace. What do you bring out in your book-length study on Saint Thomas's thought on this matter?
Based on what I have just said, it can seem rather surprising that I managed to write an entire book war and ethics in Aquinas.

There are two things I tried to do in the book.

One is show that Aquinas's perspective on the moral problem of war should not be reduced to what he does in Summa theologiae II-II, q. 40, a. 1. He addresses the use of force in other contexts within his writings. Particularly important are his scriptural commentaries. He has got quite a bit to say there. Also very important is the fact that he situates his discussion of war within the wider context of a discussion about peace. I wanted to bring the discussion of war in question 40 back to the wider horizon of peace. In some respects, Aquinas's commentators have done him a disservice because they have detached question 40, article 1, from its anchoring in peace.

The other aspect of my book is to drill down into some of the key topics that Aquinas discusses apropos war.

He gives three famous criteria or moral requirements that must be met if the war is to be considered just. The war must be declared or undertaken: 1) by the legitimate authority; 2) for a just cause; 3) for a right intention. I have separate chapters on legitimate authority and just cause. Right intention ends up getting discussed in several of the chapters.

I also look at related issues, such as the relationship of just war to the so-called “precepts of patience”, to use a term from Gratian. The precepts of patience are derived from the Sermon on the Mount. For instance, Jesus tells us to turn the other cheek. To what measure is this idea of just war compatible with the evangelical call to nonviolence in God's Kingdom? I have a chapter on that question.

I look at some issues that that have been debated in recent times: for instance, the question of preventive war. Can war be waged preventively to forestall an even greater evil?

Then, I examine what has come to be called the moral equality of combatants. This is the supposition grounding much of modern international law. When armies confront each other on a battlefield, in making judgments about the individual soldiers fighting in the war, we abstract from the cause for which they're fighting. That is an issue for their superiors. Instead, we focus on how they conduct themselves in the war: whether or not they follow the rules of war. The idea is that combatants on each side face off as equals, as it were, who are held to the same moral standards on the battlefield. I examine whether the moral equality of combatants is rooted in the thought of Thomas Aquinas.

Finally, I conclude the book with an examination of how Aquinas’s thought has been received within the contemporary teaching of the Catholic Church. I try to bring the story up to date, as it were.

The Relectio de indis...was really the first time that we find a Catholic thinker in the just war tradition examining a concrete case of use of armed force.


This post is for paying subscribers only

Sign up now and upgrade your account to read the post and get access to the full library of posts for paying subscribers only.

Sign up now Already have an account? Sign in