Thomas Aquinas’ Just War Theory

Introduction

This post recites Thomas Aquinas’ just war theory as described in Summa Theologica I-II Q. 91 and 94, and II-II Q. 40-42. His jus ad bellum is explained as well as his tentative steps in developing jus in bello criteria. A psychological interpretation of the “good intentions” criteria is given, and finally a comparison with Aristotle’s “bottom up” approach to the ethics of warfare is made. All quotations from the Summa Theologica are taken from Holmes (2005).

Grounding JWT in Terms of Divine, Natural, and Human Laws

Thomas Aquinas sets up a “hierarchy of laws” as the foundation for the ethical evaluation of warfare. This hierarchy moves from the divine (Eternal Law), through natural laws, finally ending with concrete human laws.

The Eternal Law, also called Divine Reason or Divine Law, is the operation of Divine providence. Everything is subject to such law, and it is unchangeable and eternal. (ST I-II, Q. 91, Art 1)

Natural laws are the way rational creatures participate in the Eternal Law (ST I-II, Q. 91, Art 2). Natural laws are specifically applicable to rational animals, unlike Eternal Law which has universal applicability.

Human laws are needed because we do not fully participate in the dictates of Divine Reason (ST I-II, Q. 91, Art 3). Further, human laws do not have the inerrancy possessed by the demonstrated conclusions of the natural sciences.

Why then do we even need an Eternal Law? The Eternal law directs man towards his last end, eternal happiness, but this end is inproportionate to man’s rational faculty (ST I-II, Q. 91, Art 4). Further, conflicts in human law can and do occur, because of errors in judgement. Such conflicts cannot occur within Eternal Law. Finally, human law can neither forbid or punish all evil deeds; Divine Law fills this gap and is thus needed to forbid all sins.

Statement of Just Reasons to Go to War

It is not always sinful to initiate or wage war, according to Aquinas, and he posits three conditions that must be met to initiate a just war. (ST II-II, Q. 40, Art 1)

First, war must be called for by sovereign authority. “It is not the business of a private individual to declare war, because he can seek for redress of his rights from the tribunal of his superior.”

Second, just cause is required – “those who are attacked, should be attacked because they deserve it on account of some fault.” Aquinas uses specific examples given by Augustine – to avenge wrongs, to punish a nation for refusing to make amends for the wrongs inflicted by its subjects, or to restore that which has been seized unjustly.

Third, belligerents must have good intentions. They must seek to advance the good, or to avoid evil at least. Examples of ill intentions include aggrandizement or cruelty.

Interlude: the “Dispassionate Warrior”

This third jus ad bellum criterion is a claim about the psychology of a justly acting warfighter: his goal is to “uplift the good,” perhaps in a dispassionate manner. Aquinas quotes Augustine who notes that “passion for inflicting harm, the cruel thirst for vengeance, an unpacific and relentless spirit, the fever of revolt, the lust of power, and such like things, all these are rightly condemned in war” (ST II-II, Q. 40, Art 1). Aquinas continues, removing agency from the warfighter, stating that “to have recourse to the sword… by the authority, so to speak, of God, is not to take the sword, but to use it as commissioned by another, wherefore it does not deserve punishment.”

Compare this (Kaurin, 2017) with the stance taken by one of the soldiers in Shakespeare’s Henry V: “If his cause be wrong, our obedience to the King wipes the crime of it out of us.” Aquinas goes further, replacing relief of moral responsibility with a form of technocratic detachment.

Does the concept of a dispassionate warrior make sense? To successfully prosecute a war, a certain joie de guerre is required, a passion needed for accomplishing what must be done - and a source of distress for the warfighter when he realizes he has trouble turning it off. This is the very opposite of dispassion.

Just Conduct in War

Compared to his jus ad bellum, Aquinas’s jus in bello is considerably less developed. He addresses only a few aspects of what constitutes just conduct in war: the role of religious personnel, the use of ambushes, and the ethics of strife and sedition.

Is it lawful for priests and clerics to fight in a war? Aquinas answers (ST II-II, Q. 40, Art 2) in the negative, but in defense of this position, he quotes Gregory who uses a shepherd/wolf/sheepdog analogy: “The wolf comes upon the sheep, when any unjust and rapacious man oppresses those who are faithful and humble. But he who was thought to be the shepherd, and was not, leaveth the sheep, and flieth, for he fears lest the wolf hurt him, and dares not stand up against his injustice.”

Aquinas rejects this analogy, stating that participation in warfare is incompatible with the duties of priests and clerics, because warlike pursuits are tumultuous, hindering the mind from the contemplation of divine things, prayers for the people, etc. It is unbecoming for religious personnel to slay or shed blood, says Aquinas, and “it is more fitting that they should be ready to shed their own blood for Christ, so as to imitate in deed what they portray in their ministry.” The proper way to resist an enemy of the flock is through “spiritual weapons”.

The only military tactic Aquinas mentions specifically is that of ambushes, which he states are not a form of deception but rather one of concealment. Thus, laying ambushes is a lawful activity when done in times of war. (ST II-II, Q. 40, Art 3)

Strife is described by Aquinas (ST II-II, Q. 41, Art 1) is a form of private war that is not declared by a sovereign. Because of this, it is unjust. Resistance against strife may however be without sin when it is done in self defense and with due moderation.

Sedition, the act of sewing dissent, is viewed by Aquinas (ST II-II, Q. 42, Art 1) as a sin as it is contrary to the unity of the multitude – the people of a city or kingdom, and it undermines the common good. Taken by itself, this stance would preclude both civil wars and wars of independence.

Conclusion

It is interesting to note how far Aquinas diverges from “the Philosopher” (Aristotle) on the issue of just war. Aristotle holds that it is proper for a man to engage in military training so as not to become the slave of other men, and for the same reason it is proper for the state to defend itself (Aristotle, Politics, Book 7). This is a bottom-up approach similar to what Frowe (2015) calls the “domestic analogy,” and is the opposite of Aquinas who grounds just war on Eternal, natural and human laws, and in which self defense is seen as political resistance against strife.

References

Aristotle (2020). Politics. William Ellis, trans. Independently published

Frowe, H. (2015). The Ethics of War and Peace: An Introduction. 2nd Edition. Routledge.

Holmes, A. F. (2005). War and Christian Ethics. 2nd Edition. Baker Academic.

Kaurin, P. S. (2017). “Beyond the Band of Brothers: Henry V, Moral Agency, and Obedience”. Last retrieved on 28 March 2024 from https://thestrategybridge.org/the-bridge/2017/3/1/beyond-the-band-of-brothers-henry-v-moral-agency-and-obedience