5. Thomas Aquinas (c1225-1274CE)


By the time of Thomas Aquinas in the 13th Century we find a much changed social order and Christian Church. Aquinas’s political and social situation was vastly different to any prior Christian writer on war. He was born into a society where there had been “resurrected a fundamental issue which had existed since the era of Constantine in the early fourth century, which was that of the rightful relationship between the spiritual and temporal powers. An attempt to reform the Church by purifying it from what was seen as the taint of lay corruption necessarily suggested the superiority of the spiritual power over the lay, and, indeed, it implied the right of the spiritual power to take action of some kind against recalcitrant lay rulers who would not co-operate with that programme.”[1] The Church had thus moved on a phase from the state religion to almost a religious state, it had phenomenal power and influence throughout Europe. With the Church as a major power in society in its own right, and involved in its own struggle as a political entity the time and situation seemed ripe for a re-examination of the Christian attitude to war.

Another aspect that could have informed Aquinas’ writings on war was the political situation. European society at the time of Aquinas was of a dual nature. On one level it was a developing unified nation under the Pope and Emperor. Tooke has observed that this “was an idea somewhat contrary to the facts.”[2] On a more realistic level European society was a multiplicity of cities, provinces and kingdoms with numerous rulers. However, it would seem that the writings of Aquinas on the Just War are;

“related neither to contemporary political and ecclesiastical conditions nor to the rest of his thought.”[3]

Throughout the rest of this chapter Aquinas’s thought on the Just War will be examined in the light of his received tradition and his political situation. In this way it will be seen whether Aquinas’s place in the development of the Just War doctrine is as adding a new dimension or simply restating a traditional position.


A New Format

The discussion of Just war acquires a new format in the writings of Aquinas. He takes on the format of the scholastic method, in raising objections and questions and then answering them in almost an imaginary conversation. In and of itself this is a contribution to the doctrine of Just War- it enables the reader to familiarise him or herself with both sides of the argument and analyse the Christian response. Prior to this Christian writers about war, if they noted objections, would deal with them in the main body of text- maybe making them easy to miss. It is not, however, the perfect format; the writer was able to choose his own objections and no further response was given to the answer. While the scholastic method was in the form of a dialogue it was a contrived dialogue to serve the writers own purposes.

The Justness of War

On discussing the justness of war Aquinas restates the Christian position of the undesirability of war (in his first objection);

“Now those who are threatened by Our Lord with punishment, according to Matthew 26:52 ‘All that take the sword shall perish with the sword.’ Therefore all wars are unlawful.”[4]

In responding to this objection Aquinas recalls the position of Augustine in Against Faustus Book XXII:74 (see above p.31) The justness of war is then highlighted as Aquinas discussed the requirements for a Just War. It would seem then that Aquinas merely restates the traditional position. However, his own developments and context are revealed in what he omitted. The topics of guilt and retribution that were seen as an important part of Augustine’s understanding are treated only slightly by Aquinas, in only a single sentence;

“…those that are attacked, should be attacked because they deserve it on account of some fault.”[5]

Gill observes that;

“In part, (these omissions/changes) reflect a radically changed social context, in part a very different religious psychology, and, in part, a culture which did not regard war as presenting particularly serious ethical problems.”[6]

While Aquinas’ thinking was not new, the way in which he treated the subject and the areas he chose to focus on reflected the changes in society since Augustine. War was now an accepted and almost integral part of Church life, The Crusades and political power of the Papacy had made this possible. This is not a development of the Just War doctrine, rather it reflects the position of church in society and how attitudes to war in general had changed.


Right Authority

Aquinas declares;

“Three things are required for any war to be just. The first is the authority of the sovereign on whose command war is waged. Now a private person has no business declaring war…”[7]

His reliance on tradition is evident in his elucidation of this point; he quotes Augustine in Against Faustus Book XXII:75 (see above p.31) verbatim.

In examining the social context of Aquinas, and its influence on his writings one must examine it on two levels- the philosophical and the political. Aquinas, seemingly, was a great advocate of natural law morality. Tooke observes that he

“accepted the psychology of the morality of his time For Aquinas, the essence of human nature was its rationality, and this he believed was the seat of morality.”[8]

The nature of human beings was therefore carnal and needed to be kept in check. Therefore, human nature by itself could not be trusted to determine the justness of a course of action involving violence. In assuming this morality, Aquinas differentiated between individual and public action. “This assumes personal revenge is likely to be more savage than social, and that social morality is higher than personal.” Aquinas “kept apart private evil and public evil… It hardly needs saying that here Christ overwhelmed Christ’s simple words with natural law morality… (T)throughout he assumed the words were spoken to individuals not to societies.”[9] It would seem that only societies can use force without violating human nature. Porter notes that;

“The communal contexts of morality are of central importance to Aquinas because he sees the basic institutions of society as embodying the principles of justice, which guarantee right relations among individuals and between individuals and the community.”[10]

However, on closer analysis, it would seem that even though Aquinas’ philosophy is couched in the terms of natural law morality it is purely a restatement of Augustine’s previously stated view. He may not have used the same language or philosophical background but the separation of public and private was clearly evident.

The political context of Aquinas would seemingly have provided him with the opportunity to develop this aspect of just war thinking further. With Ambrose, his predecessors, the Pope (or Christian Bishops) as a legitimate authority in time of war would not have been considered. Indeed, Ambrose clearly recognises whose concern war was;

“If we have to consult concerning military affairs, the opinion of a man experienced in warfare should be waited for, when the question concerns religion think upon God.”[11]

Augustine developed this possibility in allowing God the authority to declare war to God, but the Church’s position as a political power that pervaded all aspects of life and society was nowhere evident in the writings of Aquinas.

Aquinas ignored social and political conditions in constructing his writings on war, preferring to rely on received tradition. The question of who held the appropriate political authority to declare war “was a rather delicate question….in a society so abounding with princes and petty rulers.”[12] Aquinas missed an opportunity to develop just war thinking in the context of legitimate authority, by not applying his teachings to the social conditions of his time. The authority of princes and petty rulers in relation to the Empire, as well as the authority of the Church could quite conceivably have been questions that occupied those Christians wanting to go to war, or even just considering the wars of the period.

The only seeming development that is made by Aquinas with regard to right authority is when he is discussing the question; ‘Whether sedition is always a mortal sin?’ He outlines the argument of Augustine and then concludes;

“Wherefore it is evident that the unity to which sedition is opposed is the unity of law and common good whence it follows manifestly that sedition is opposed to justice and the common good. Therefore by reason of its genus it is a mortal sin…”[13]

 He agrees with this in a general sense but clarifies his thinking in a way that would maybe make Augustine balk. He argues that to fight against a tyrannical government is just because it

“…is not just, because it is directed, not to the common good, but to the private good of the ruler, as the Philosopher (Aristotle) states. Consequently there is no sedition in disturbing a government of this kind, unless indeed the tyrant’s rule be disturbed so inordinately, that his subjects suffer greater harm from the consequent disturbance than from the tyrant’s government. Indeed it is the tyrant that is guilty of sedition, since he encourages discord and sedition among his subjects…”[14]

Bourke observes Aquinas “was not entirely in agreement with some… aspects of Augustine’s thought”, and that he “feels that he has to present and somehow defend St. Augustine’s views, but he also takes care to introduce various explanations from the Aristotelian tradition.”[15] In his thinking on war, however, this is a surprising and welcome departure from received Christian tradition, even if it is a development from received philosophical tradition. This is, however, a development of the doctrine of just war that reflects the developed position of Christianity. No longer bound by the constraints of supporting the Empire, the Church is now able to speak freely. Aquinas takes the thinking of Ambrose (who said that acts by the Emperor could be wrong) a step further by saying that an act against the Emperor/ or equivalent could be right. This could be a product of the political conditions of his time, but in the light of his other writings is more likely to be a pragmatic development, as a result of the social situation of the Church and lessons from history.


A Just Cause

“Secondly, a just cause is required, namely that those who are attacked, should be attacked because they deserve on account of some fault.”[16]

Again, this is a seeming restatement of the received and accepted Christian tradition. In support of this argument he cites Augustine;

“Wherefore Augustine says ‘A just war is wont to be described as one that avenges wrongs, when a nation or state has to be punished, for refusing to make amends for the wrongs inflicted by its subjects, or to restore what it has seized unjustly.’”[17]

This statement of just cause does not seem to lay down the same restrictions as those given by Ambrose and Augustine. He makes no mention of the purpose of a just war so important to his predecessors. The knowledge that a just war was fought to obtain peace placed restrictions on the actions and attitudes of the participants. There is no attempt by Aquinas to reconcile the teaching of Jesus to “turn the other cheek” ( see Matt 5:39) with this seeming retaliatory attitude in a just cause. It would seem that for Aquinas the ends justify the means, in fighting for the common good that purpose supersedes any peaceful motive. This can be seen as a reflection of the prevalence and acceptance of war in European society in the thirteenth century. War was so accepted and almost “the norm” that for Aquinas to have legislated any further would not have been pragmatic.


A Just Intention

“Thirdly it is necessary that the belligerents should have a rightful intention, so that they intend the advancement of good, or the avoidance of evil.”[18]

Again, Aquinas quotes Augustine as his authority and then concludes;

“For it may happen that the war is declared by the legitimate authority, and for a just cause, and yet be rendered unlawful through a wicked intention.”[19]

Aquinas simply restates the already accepted Christian traditions. At a time when Imperial ambitions were waning, he was, perhaps, safe to write these things. However, he does not seem to fully develop the reasons for war behind the intention. At no point are contemporary social and political conditions called upon to discuss the theory and practice of war. One is struck in Aquinas by the lack of application of his principles for explanation and example. When reading Ambrose and Augustine contemporary and historical examples are evident in reading their discussion of war. On examining the writings of Aquinas the reader would be left wondering whether his life had been free from any contact with war.


Conduct of War

While not being one of Aquinas’ three prescribed conditions for a just war, he does cover the conduct of war at certain points in his discussion of war. However, for the most part he simply reiterates the words of Augustine from Against Faustus Book XXII:74. (see above p.34) Rather than developing any thinking of his own and responding to any technical developments that had taken place Aquinas simply restates the position of Augustine. However, in discussing ambushes Aquinas develops some interesting and innovative thinking;

“… a man may be deceived by what we say or do, because we do not declare our purpose or meaning to him. Now we are not always bound to do this, since even in the Sacred Doctrine many things have to be concealed, especially from unbelievers, lest they deride it, according to Matt 7:6 ‘Give not that which is holy to the dogs.’ Wherefore much more ought the plan of campaign to be hidden from the enemy. For this reason among other things a soldier has to learn is the art of concealing his purpose lest it come to the enemy’s knowledge, as stated in the book on Strategy. Such like concealment is what is meant by an ambush which may be lawfully employed in a just war.”[20]

While he again cites Augustine, uniquely for Aquinas he adds to the arguments using both ‘The Book of Strategy” and the New Testament. He gives a slightly deeper insight into the reasons for this justification- giving the impression that this is the result of independent thought and study rather than a pure reliance on received tradition.

The Participation of Clerics

Following Augustine’s allusions to the subject of warfare and the clergy, Aquinas returns to the subject. As a cleric that had been free from participation in warlike matters through his life Aquinas was able to assume the mantle that had been left to him by Ambrose. In a similar vein to Ambrose he declares war as a pursuit alien to the clerical office.

“On the contrary, It was said to Peter as representing bishops and clerics (Matt 16:52) ‘Put up again thy sword into the scabbard.’ Therefore it is not lawful for them to fight.”[21]

It seems strange to the reader that Aquinas uses the same passage as he used earlier to justify authorised warfare to outlaw warfare. His further discussion of the participation of the clerics raises the question as the validity of war for a Christian at all;

“(A)ll the clerical Orders are directed to the ministry of the altar, on which the Passion of Christ is represented sacramentally… For this reason it has been decreed that those who shed blood, even without sin, become irregular. Now no man who has a certain duty to perform, can lawfully do that which renders him unfit for that duty. Wherefore it is altogether unlawful for clerics to fight, because war is directed to the shedding of blood.”[22]

This statement would not seem out of place alongside the writings of Tertullian and the other early Church Fathers who outlawed war. In discussing the incompatibility of warfare with the flesh and blood of Christ, Aquinas is merely reflecting the tradition of the Church that those who killed in warfare must abstain from communion for a period of time. It does, however, raise the question that if someone is not ‘worthy’ to receive communion should he not abstain from the thing that makes him unworthy? If adultery would deny someone communion for a period of time, no one would say that because the person can take it again after this time has passed that adultery can be ‘just’ in some cases. This is not a deficiency of Aquinas, rather it is a tradition (which some may see as a deficiency) of the time that is reflected in the writings of Aquinas. While Aquinas subscribes to the ideas of Ambrose he is able to take them a step further because of this development in Church practice- thus he reflects his historical context in this aspect of his discussion of the just war.

A further development of Aquinas’ has its roots in Augustine declaring war on the Donatists. Since then, and increasingly so up until the time of Aquinas the Church’s power over declaration of wars and encouraging people to participate in them had continued to grow. Aquinas incorporates this ‘accepted’ state of affairs into his writings on the just war;

“Wherefore it is the duty of clerics to dispose and counsel other men to engage in just wars.”[23]


The influence of Ambrose manifests itself plainly when one examines the just war thinking of Augustine. Ambrose’s treatment of the just war had been scattered, and arose in response to questions and situations as they arose- the reader needs to search his writings to extract a coherent, and, recognisable code for the just war. For, Augustine, however, his treatment was much more systematic and thoughtful. This, itself, was a reflection of the political and social climate of his time. He had to answer the charges laid at his (and Moses’) door by Faustus, while reconciling this with his pacifist heritage in a time of decline for the Roman Empire. In selecting and using his examples Augustine tries to create the impression that war has always been part of the Christian tradition when conducted in accordance with the code for a just war. The most startling departure from the thought of Ambrose is in reference to the declaration of war by God. Here, Augustine harks back to the time of the Old Testament when God was involved in the defence of Israel. This is probably the greatest reflection of the situation in which Augustine found himself in. In defence of Christianity, he had sanctioned force against the Donatists, this was not a secular act, it was a spiritual one (given secular sanction) and it needed defending with reference to the Christian tradition.

The acceptance of warfare led to certain questions being glossed over. As war became more accepted within Christian society it was no longer necessary to discuss wars of defence. Nor was it necessary to discuss the non-participation of the clergy, this was an accepted fact.

It is still true to say that Augustine is the foremost authority on just war because he is the one to whom people make reference. But this position is not necessarily justified, on examination of his writings on war, apart from the minor adjustment and addition he restates the position that had been adopted by Ambrose. Indeed, in some areas of just war thinking (especially the causes of a just war) it is much more enlightening and informative to read Ambrose. This is, in itself, a reflection of the new position in which Christianity found itself- war was an accepted part of their life.

The influence of Augustine can be manifestly seen in an examination of the writings of Aquinas. To restate the position of Tooke, in all his requirements for the just war and its conduct

“Aquinas’s direct teaching… is slight and unoriginal. Devised more or less wholesale from Augustine… its is abstract and theoretical, and inspired by no personal emotion or thought. It is related neither to contemporary political and ecclesiastical conditions nor to the rest of his thought.”[24]

It would seem that Aquinas “as a theologian and a metaphysician considered war from an individual point of view rather than an objective and social one.”[25]

In borrowing from Augustine Aquinas left the task half finished. In his discussion of war he did not differentiate between offensive and defensive war, he omitted to say that war should be a last resort following arbitration, and the purpose of war was not considered. Indeed, Aquinas’ place as a thinker on war should be questioned, he offered only a couple of minor developments, and no new insights into the just war and its conduct for Christians, he took none of the opportunities that his new position offered him (save the instruction that clerics could promote war and the possibility of just sedition). Times had changed since Augustine yet Aquinas found no necessity to clarify or change his thinking. The largest example of this flaw can be found in him allowing “authority to declare war to ‘any public person’, well knowing the superiority of the imperial and papal power in his day.”[26] In saying this it may seem a little harsh, but from the examination of his thinking there was no real development from received tradition, neither were there any historical examples to amplify his thinking, the like of which could be found in Ambrose and Augustine. In defence of Aquinas, Tooke offers him a lifeline;

“It is hard to believe that Aquinas could have escaped all knowledge of war in his own day even had he wished to do so. His ignoring the arbitrational activities of the papacy may say something for his honesty as well as for his prudence, for it was only just before his death that the empire and the papacy came to terms… Aquinas may have found the evidence too confused for immediate summary.”[27]

This is, indeed, a possibility but one that does not negate the fact that Aquinas’ thinking on the just war was largely borrowed from received tradition, and owed little or nothing to social or political developments or the situation in which Aquinas now found himself. Times had changed and thinking on war for the Christian required more than a regurgitation of received tradition, unfortunately, from Aquinas this is all that was received.

[1] Malcolm Barber The Two Cities. Medieval Europe 1050-1320 (Routledge,1992) pp85-86.

[2] The Just War in Aquinas and Grotius p15.

[3] Ibid. p170.

[4] Summa Thelogica II.II Question 40. Article 1. Objection 1.

[5] Ibid. Quesion 40 . Article 1.

[6] Robin Gill A Textbook of Christian Ethics (T&T Clark Ltd. 1985) p317.

[7] Summa Theologica II.II Question 40

[8] The Just War in Aquinas and Grotius p96

[9] Ibid. pp122-123.

[10] Recovery of Virtue. The Relevance of Aquinas for Christian Ethics (SPCK 1994) p135.

[11] Letters XVII:7

[12] Joan D. Tooke The Just War in Aquinas and Grotius p26.

[13] Summa Theologica II.II Question 42. Article 2.

[14] Ibid. Question 42. Reply to Objection 3.

[15] Vernon J. Bourke Aquinas’ Search for Wisdom (Bruce Publishing Company, 1965) pp70-71.

[16] Thomas Aquinas Summa Theologica II. II Question 40. Article 1.

[17] Ibid. Question 40. Article 1.

[18] Ibid. Question 40. Article 1.

[19] Ibid. Question 40. Article 1.

[20] Ibid. Question 40. Article 3.

[21] Ibid. Question 40. Article 2.

[22] Ibid. Question 40. Article 2.

[23] Ibid. Question 40. Article 2. Response to Objection 3.

[24] Joan D. Tooke The Just War in Aquinas and Grotius p170.

[25] Ibid. p28.

[26] Ibid. p170.

[27] Ibid. p28.