Philosophy of War: 3 Influential Theorists

What were St Thomas Aquinas’, Sun Tzu’s, and Carl von Clausewitz’s philosophies of warfare?

Sep 29, 2023By Luke Dunne, BA Philosophy & Theology

philosophy of war influential theorists


This article attempts to introduce the philosophy of war through an overview of three very important military theorists: Sun Tzu, Thomas Aquinas, and Carl Von Clausewitz. As we will see, what they shared was a concern with providing some structure to the apparent chaos of warfare. This article begins with a discussion of the philosophy of war as a whole, and poses the question of what it is to take a “philosophical” approach towards war. It then moves on to explain some of the core elements of Sun Tzu’s, St Thomas Aquinas’, and Carl Von Clausewitz’s conception of warfare.


Defining the Philosophy of War

battle canal du nord
Photograph of German Prisoners of War by William Rider-Rider, 1918, via Imperial War Museums.


The philosophy of war is concerned with bringing a range of philosophical methods to bear on the problems presented by military conflict. Sometimes, this means theorizing the conditions under which war is right, or the conditions for proper conduct in times of war. For other theorists, this means applying philosophy to the strategic and tactical problems of warfare.


Attempting to make sense of the chaos of warfare using philosophy might sound self-indulgent to some, but to others, philosophy in any form is always an attempt to offer conceptual clarity to undifferentiated, fluid, random, diffuse processes. In this view, the point of philosophical abstraction is a way of attempting to make sense of things that are otherwise difficult to grasp. Philosophy offers a simulation of reality that we can make sense of, in the same way (or in an analogous way) as how a diagram of a battlefield makes sense of an otherwise frightening, entropic event.


Yet philosophy is also concerned with addressing abstractions, discharging them, and finding particular referents for them. Whether the philosophy of war (especially the strategy-orientated parts of it) justifies its worth on the battlefield is a matter for generals and military historians to determine.

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1. Sun Tzu: The First Military Theorist

sun tzu statue
A bronze statue of Sun Tzu, via NDU Press.



Sun Tzu was a Chinese theorist of war. The major work attributed to him, the Art of War, is beloved by generations of soldiers as well as politicians, corporate executives, and others who conceive of themselves (however erroneously) as being well-placed to learn important practical lessons from ancient warfare.


Although not much is concretely known of Sun Tzu’s life, we know him to have been a general and strategist for King Helü of Wu, a ruler who made several successful conquests during the turn of the 5th century BC. Sun Tzu’s specific successes as a general are not well recorded, and the most famous story about him is probably apocryphal, but worth hearing in any case.


King Helü attempted to test Sun Tzu by having him organize the king’s concubines into a company of soldiers. When they failed to take their work seriously enough, Sun Tzu executed the two most favored concubines, whom he had appointed company commanders. The king protested, but Sun Tzu waved him off, claiming that, having been appointed, a general must do his duty even against the king’s wishes. After his practical education in warfare, Sun Tzu wrote his famous book.


eastern han three kingdoms battle
Eastern Han—Three Kingdoms Battle Painting, photographed by Gary Todd, 2012, via Flickr.



Various features of the Art of War bear particular attention. For one thing, it is an extremely systematic work. It begins with a categorization of all of the various factors which contribute to the success or failure of a military campaign. Sun Tzu advocates a holistic assessment of these factors—the weather is just as important as a general’s competence or incompetence. Moreover, Sun Tzu stresses the significance of elements of warfare that seem strikingly modern—in particular, his discussion of the economics of warfare and of the role of intelligence.


Sun Tzu advocates a mixture of conservativism and competence. He believes one should advance from a position only when it has been completely secured, and advocates the near-sacred duty of command hierarchy. However, he also emphasizes the significance of flexibility both within the military apparatus and in terms of how the war is prosecuted.


The cynical view would be that Sun Tzu’s influence, especially among Westerners, comes partly from the impunity with which his work has been interpreted to fit almost any conflict situation and to justify any cause of action. Of course, this is not necessarily a mark against Sun Tzu himself—it is difficult to write a theoretical work that is both ambitious and, at the same time, idiot-proof.


2. St Thomas Aquinas 

st thomas aquinas
St Thomas Aquinas by Sandro Botticelli, 1482, via Providence College.


St Thomas Aquinas was a philosopher of war in quite a different way to Sun Tzu, concerned not so much with what it takes to win wars as with what it takes to justify them.


It is worth saying at the outset that what seems to be a sharp distinction of purpose between Aquinas’ work and that of Sun Tzu (and Clausewitz, who we will get onto shortly) does not translate into totally opposite methods of approaching war. Of course, Aquinas is less troubled by the practical business of how to win wars, yet is equally concerned with what it is to offer a complete description of war, to characterize the conditions of war, and to determine what is required for war to end.


Aquinas’ theory of “just war” has two elements—jus ad bellum (the right to begin war) and just in bello (right during wartime). According to Aquinas, war is justified on the basis that it is called by a sovereign authority, that it has a just cause, and that those who wage the war have morally right intentions.


It is worth pausing to observe how stringent these conditions were in a medieval context, when questions of right intention and just cause were rarely, if ever, honestly considered before war was undertaken. Yet there is a conservative strand in Aquinas’ formulation of jus ad bello—the first condition. It states that war can only be justly prosecuted by the sovereign, and is a transparent attempt to guard against war being waged by individuals, including against the sovereign.


leviathan hobbes min
Leviathan Frontispiece by Abraham Bosse, 1651, via Columbia University.


Equally, the idea of a just cause is sufficiently broad to justify many wars if we are to take the aggressor’s perspective. In particular, the conditions of a just cause, including righting a past wrong and restoring that which was unjustly taken in the past, can more or less accommodate any war of aggression.


This is a problem for Aquinas, given that he at no point attempts to offer a framework for making these determinations, and this lack of guidance with respect to these practical judgments substantially dilutes the constraints which Aquinas might otherwise be thought to have placed on warfare.


Yet certain elements of Aquinas’ theory of just war have proven to be extremely influential, and continue to be so today in part because they do constitute genuine constraints on the prosecution of wars. In particular, Aquinas’ discussion of self-defense as a justification for warfare has been developed extensively in the field of jurisprudence and international relations.


3. Carl Von Clausewitz 

Portrait of Carl Von Clausewitz by Karl Wilhelm Wach, 19th century, via Wikimedia Commons.


Carl Von Clausewitz is arguably the most famous modern theorist of war. His practical experience of war came during the Napoleonic period, when Clausewitz fought for the Prussian military against Napoleon’s France. He was taken as a prisoner of war after the Battle of Jena, during which the Prussian-Saxon forces were authoritatively beaten. The subsequent, involuntary alliance with France led Clausewitz to leave the German army, after which he dedicated time to his major work of military theory, On War.


Part of Clausewitz’s theory of war was the view that, despite the appearance of chaos, every aspect of war could be explained with reference to deep structural factors, including the economic and social context which surrounds it. It is Clausewitz’s conception of war as an all-consuming enterprise—one which can both be explained by holistic social analysis and which can directly involve the whole of society—that has led many subsequent interpretations to focus on his work as inaugurating the modern era in military theory.


This impression of Clausewitz as the first modern military theorist was aided by the subsequent reliance of later generations of Prussian leaders on insights gleaned from him, given that by the late 19th century, Prussia had a claim to be the most modern military apparatus in the world. Yet for all of Clausewitz’s emphasis on the deep, structural forces which drive success or failure in warfare, he was nonetheless keenly aware of the uncertainty involved in warfare: he was particularly skeptical of the value of wartime intelligence and stressed the significance of irrational, emotional states in determining the course of wars. Deciding when predictive judgments can and cannot be made is one of the most difficult tasks for philosophers of war.

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By Luke DunneBA Philosophy & TheologyLuke is a graduate of the University of Oxford's departments of Philosophy and Theology, his main interests include the history of philosophy, the metaphysics of mind, and social theory.