247. Onward, Christian Soldiers: Just War Theory

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Aquinas follows medieval legal thinkers in defining the conditions under which war may be justified, and proposes his famous doctrine of double effect.



Further Reading

• P. Contamine, War in the Middle Ages, trans. Michael Jones (Oxford: 1992).

• J.T. Johnson, Ideology, Reason and Limitation of War: Religious and Secular Concepts, 1200-1740 (Princeton: 1975).

• J.T. Johnson, The Just War Tradition and the Restraint of War (Princeton: 1981).

• G.M. Reichberg, Thomas Aquinas on War and Peace (Cambridge: 2016).

• F.H. Russell, The Just War in the Middle Ages (Cambridge: 1975).

• J.D. Tooke, The Just War in Aquinas and Grotius (London: SPCK, 1965).

• P.A. Woodward (ed.), The Doctrine of Double Effect (Notre Dame: 2001).

Stanford Encyclopedia: The Doctrine of Double Effect


Thomas Mirus on 17 February 2016

I don't think it's fair to

I don't think it's fair to say that the Crusades as a whole (as opposed to atrocities committed in the course of them) were hypocritical, or that any warfare cannot be reconciled with Jesus' words. "Turn the other cheek" does not forbid self-defense in every circumstance, much less using violence to defend others towards whom one might have an obligation. And using a sword in legitimate self-defense is not necessarily "living by the sword." (Especially because the latter words were said in the context of Jesus willingly giving himself up, the necessity of which He had made clear to the Apostles on multiple occasions.)

In particular, states and those running them have obligations that individuals do not. Individuals have the "luxury" of pacifism and attaining to the higher ideal of eschewing even legitimate self-defense, but the leaders of states do not because of their responsibilities to their citizens. Tertullian (who became a heretic) notwithstanding, the Patristic tradition does not condemn defensive war. St. Paul himself said the government could use violence legitimately in punishing criminals.

As for Fr. James in Calvary (which I liked on the whole), the word used in the commandment against killing is generally held to be a word for murder specifically. There may not be an asterisk, but in context it is obvious that it is not a prohibition against all killing since the Jewish law also made plenty of allowance for the death penalty.

In reply to by Thomas Mirus

Peter Adamson on 17 February 2016

Just war

Right, I think you are actually echoing some of the points made in the texts covered in this episode, in particular the point about self-defense though they also distinguish between states/rulers and private citizens. I still find it hard to see any way of justifying the crusades in general (note the intellectual contortions that were required to present them as acts of "self-defense") and of course it's not easy to disentangle the crusades from the atrocities, especially with the fourth crusade which was pretty much nothing but an atrocity, and committed against Christians in fact. More on that later when we get to Byzantine philosophy, since the fourth crusade led to the sack of Constantinople.

In reply to by Peter Adamson

Dan Gin on 15 March 2022

I'm curious to hear what the…

I'm curious to hear what the 'intellectual contortions' are exactly. I'm no philosopher, theologian or historian so somebody can correct me but my understanding is that there are around 100 passages in the Old Testament where God explicitly commands people to be killed. Even in the New Testament, Jesus explicitly says that he came to bring the sword. Old Testament warriors like Judah Maccabee were held in high esteem during the period and the Church had an extremely long tradition of blessing kings and even wars (this long predated the Crusades). I don't think that a medieval Christian would see war as something which was so incompatible with his morality that they needed intellectual contortions to justify it.

I also think that, as far as wars go, they don't get more justified than the Crusades from the perspective of self-defence of Christianity. The Seljuq Turks had invaded and occupied most of Asia Minor, the heartlands of the Eastern Roman Empire. The Seljuq capital, Nicaea, was extremely close to Constantinople. Its totally possible that Constantinople would have fallen centuries before it did without the Crusade (although the danger to it is disputed by modern historians). The First Crusade was specifically in response to a call for help from the Byzantine Emperor. By this point, around two thirds of what used to be Christendom (roughly the Roman Empire) had been conquered by Islamic armies. Three out of the five holiest cities in Christendom (Jerusalem, Antioch, Alexandria) were ruled by non-Christians. Another one of them was being threatened. It wouldn't take much for that to be classified as self-defence without resorting to intellectual contortions.

In reply to by Dan Gin

Peter Adamson on 15 March 2022

Justifying war

Well, I guess the main thing to say here is that behind all your examples is the concept that it would make sense to defend "Christianity" or "Christendom," as if that were something that could be attacked, and need to defend itself. As compared to, say, France. While we may find that a natural concept now (at least, natural to apply to the medieval mindset) it is an interesting question how early, and why, it became a notion that could be deployed in this kind of political context. I would hazard a guess that it was actually a product of the Crusades - like, they discovered "Christendom" as something that could go off to war only because they wanted to go off to war. But in any case that presupposition was itself, if not an intellectual contortion, a major intellectual development or invention. Effectively it amounted to saying that any Christian military force can go kill anyone who attacks other Christians, anywhere on earth; a pretty bold concept that, as I say, has its own history.

Also it would be remiss of me not to point out the irony of your invoking defense of Constantinople as a rationale given that when it in fact fell and was sacked, that was at the hands of the Crusaders. But of course that is much later in the story than the initial justification of the Crusades.


In reply to by Peter Adamson

Dan Gin on 15 March 2022

Holy War Preceded Crusades

It is definitely true that the Crusades played an important role in the development of many aspects of medieval Christendom, from artistic movements to the economy (for example, the international banking system run by the Templars). However, (and please correct me if I've misinterpreted your statement) it is certainly not the case that Christendom was created as a result of the Crusades. The whole idea behind the papacy was as the divinely appointed leader of a community of believers. This doesn't mean that it was a monolith which never opposed itself (quite the opposite), but Christendom had basically always been seen as something 'real'. Indeed, much more real than France.

Crucially, it was also seen as something which it was the responsibility of warriors to protect. Constantine the Great declared himself the protector of all Christians in non-Roman lands. Justinian justified his reconquest of the West as a way to crush the Arian heretical barbarian kingdoms. Byzantine emperors would call themselves protectors of Christians living under Islamic law even during the Crusade period. Charlemagne was seen as the defender of the Church and the Christian West against "pagans and Saracens" (that was in his coronation as emperor). The argument that the "Crusades initiated the idea of holy war in defence of Christendom" is thus untrue.

In reply to by Dan Gin

Peter Adamson on 16 March 2022


Ok right, those are good examples and I find that pretty convincing; especially your point that rulers appointed themselves defenders of Christians who didn't evne live in their territories. I think it is still a significant change that comes only in the Crusades that a whole foreign war would be launched on behalf of Christendom, but of course that is part of what makes the Crusades interesting.

The role of Byzantium in this is interesting: you're right to say that the first crusade was to some extent initiated by a call for help from the Eastern emperor but for most of the history of the Crusades the Byzantines were looking on in horror and dreading the appearance of crusaders... with good reason, as it turned out. So that already shows how problematic it is to assume that there was an agreed notion of "Christendom" as anything like a political unit that could launch a war the way a kingdom might do.

But I think (and this is my fault) we have gotten off my main point, at least in the original podcast, which is how the medievals sought to justify violence within a Christian ethical system in the first place; the Crusades were not exactly "turn the other cheek" after all. This is hardly an original thought on my part of course but I think it is an important thing to bear in mind when one looks at medieval literature on just war theory: that they are wrestling with a contradiction between religious injunctions to avoid violence and political imperatives to engage in it. As you said in your original post one can find exhortation to violence in the Bible too, but that doesn't make Christ's more pacifist statements just go away (especially since many such passages are in the Old Testament).

In reply to by Peter Adamson

Dan Gin on 17 March 2022

Fair enough. This is a…

Fair enough. This is a terrific podcast and I hope I can buy the book form at some point.

Bear on 26 February 2016

Liberation of Jerusalem

Just a correction to a date - the Liberation of Jerusalem occurred on 15 July 1099, rather than 13 July.

Ça va sans dire that the incidents mentioned - the Liberation of Jerusalem, the Sack of Constantinople and the Albigensian Crusade - are much more complex than is usually portrayed.

For the Sack of Constantinople suffice it to observe that (a) the Franks arrived in Greece at the time of a civil war, (b) one of the contenders made promises to the Franks he could not possibly keep, and (c) the Doge of Venice was blinded by the Byzantines many years before and he was a little put out by it.

The Albigensian Crusade had less a religious purpose as a political purpose of extending the Royal Demesne to Languedoc. At the time, although the different parts of France, Languedoc, Aquitaine, Normady, Champagne, Burgundy &c, were theoretically suzerain to the King of France, in reality they had much independence. So often the kings of France looked for reasons to extend the Royal Demesne.

One should note that the violence is often emphasised in these events, and this is the thing that shocks most modern people because they do not see it. However, modern warfare is even more violent, and if one considers "enhanced interrogation" then today governments are as violent as in the middle ages.

In reply to by Bear

Peter Adamson on 28 February 2016


Thanks for the correction on the date - I guess it was a several day siege ending with the breakthrough on the 15th. I'll correct that for the book version.

See below in my answer on the next comment for further, more substantive discussion.

mehmet on 27 February 2016

I think there is a tension

I think there is a tension within christianity itself. Old testament advocates "an eye for an eye". New testament advises "turn the other cheek." And christianity accepts both as the word of god. But I think the old testament fitted much better to the temperaments of germanic peoples and vikings who became masters of western europe after the roman collapse. Hence we have the strange phenomenon of medieval catholic bishops riding to war in full armour..

Orthodox christianiy, on the other hand, had made a different choice. Even though they were under much harsher military pressure (sassanids, slavs, arabs, turks, and fellow christian crusaders) their priests and bishops successfully resisted the urge to pick up arms (though they went to battle to encourage the troops)..

In reply to by mehmet

Peter Adamson on 28 February 2016

Christianity and violence

Yes, I think that's basically right though I don't know enough about the Byzantine context to agree or disagree with the last part. To me the tension is precisely what is interesting here, since it generated the complicated discussions covered in this episode. This is also what I would say in response to the previous comment: obviously (as I said in the episode) the medieval Christians had no monopoly on violence, the interesting part is that they themselves gave such intense thought to the question of how violence could be justified within a religious framework. And by the way some of the atrocities that occurred during the Crusades - and I can see no reason to deny that they were atrocities, whatever the historical background that gave rise to them - did also occasion shock and revulsion even at the time. Again, I'd recommend the History of the Crusades podcast to listeners who want to learn more about the events themselves, which obviously are not really the focus of my series.

In reply to by Peter Adamson

Bear on 2 March 2016

This discussion has continued

This discussion has continued to modern times, and has be changed in many ways.

I know that it is a huge topic, but there were two lacunae which would make the discussion much more nuanced. The first is the consideration of success. If a war or uprising had little or no chance of success, then it was not considered licit to pursue it. War involves the taking of lives, the maiming of bodies and the destruction of property, all of which are evil. Consequently, if there was no chance of success, then pursuing armed conflict would only increase the evil. This lead to the moral accountants getting out their abaci and spending months and years debating the ethical calculus of war, particularly when they combined it with Double Effect.

The Easter Uprising in Dublin in 1916 (its centenary is fast approaching), was initially condemned because it had such little chance of success. However, this condemnation was quickly forgotten as the leaders of the uprising, Pearce and his companions, were murdered by the British.

The second concerns what is now called "collateral damage", but is more accurately described and dead and injured civilians. Thomas was coming from a position that it is never permitted to kill the innocent, and armies had to take care to ensure that they didn't. At the risk of sounding like a Sinn Féin shill, in the guerilla warfare after the Easter Uprising waged by Mícheál Ó Coileáin, the focus was on the occupying forces - so the British Army, the Royal Irish Constabulary and collaborators were targeted. For Ó Coileáin it probably was pragmatism, but for the leader, Éamon de Valera, it was fighting morally.

Just War Theory has had some influence in modern times also. One eminent 20th Century Philosopher, Elizabeth Anscombe, certainly adhered to Just War Theory. When Oxford University granted Harry S. Truman an honorary DPhil, Anscombe objected very strongly. Her reason was that Truman had ordered the bombing of Hiroshima and Nagasaki knowing full well that huge numbers of civilian deaths would occur. Thus, he was directly responsible and culpable for the murder of thousands of people.

The discussion of recent geopolitical events certainly takes a different slant when viewed through the lens of Just War Theory.

In reply to by Bear

Peter Adamson on 2 March 2016

More just war

Thanks, that adds a very interesting perspective. I agree that the idea of attainable goals in war is important, but I actually don't recall coming across that criterion in the medieval sources I was looking at.  By contrast, "collatoral damage" has of course become a more urgent issue with modern weapons, but was already considered and discussed by the medievals, as I think I said in the episode. For instance the Christians killed along with heretics in the crusade against the Cathars.

In reply to by Peter Adamson

Bear on 10 March 2016

Attainable goals was probably

Attainable goals was probably considered more by prudence rather than justice: only a fool would willingly enter a battle he knew he would you. So this is likely a later synthesis, but I would not think outside the mediaeval mind.

In reply to by Bear

Declan Foley on 13 March 2016

Ireland and war of Independence

Michael Collins was pertubed at having to send men out to kill other men.

Daniel on 8 May 2018


As much as I hate the Crusades, I don't think it's fair to call them unprovoked. Their butchering of Jews in Europe certainly was unprovoked, as were the numerous atrocities throughout, but the overarching goal I don't think was. Muslims had taken the land militarily. That provoked a Christian response attempt to regain the land.

In reply to by Daniel

Peter Adamson on 8 May 2018


Well, I guess it depends on what you mean by "provoked" - and it also may depend on which crusade we are talking about (I mean, don't get me started on the fourth crusade). But if we think about the initial crusades, obviously you're right that the military conquest of early Islam created the "need" for the crusades, but that was a seizure of the Holy Land (and much else) from the Byzantine empire, not from Latin Christendom. So you are kind of buying into the idea that Christendom and Islam are two monolithic political entities, here, aren't you? Otherwise, how does a French king get to be "provoked" to launch a war on the basis of Constantinople losing jurisdiction over Jerusalem, which by the way happened when the French king was not even a gleam in his father's eye? I know this sounds pedantic but it is relevant in this context because we are talking about technical notions of reasons for just war, and I don't think that these were present in the Crusades, even on the usual medieval understanding of just war - which didn't stop them from trying to stretch their own definition to cover the case, as I explain in the episode.

Actually we're just about to go over the Arab conquests again from the other side of the story, when we get to Byzantine philosophy!

Zarl on 1 May 2019


I'm surprised there was no mention of how the Crusades were framed as an "armed pilgrimage". In fact one of the causes of the First Crusade was that the Seljuk Turks, who had just conquered Jerusalem from the Arabs, banned Christians from the Holy Land, thus making pilgrimages to Jerusalem impossible. And about that, the notion that the Crusade was purely "offensive and unprovoked" is astonishing. Aside from the closing to the Christians of the Holy Land (which had of course originally been taken from the Christians in the first place), the Turks had recently invaded and  conquered almost all of Anatolia and laid siege on Constantinople, prompting the Byzantine emperor Alexios I to ask the Pope for help (a Byzantine plea for help to the West that would often be repeated until the final fall of Constantinople, and this despite episodes such as the 1204 sack). Add to that the reports of atrocities committed by the Turks in Christian lands, and you'll be hard pressed to find a better justified military intervention in our own times.

I understand however that there's a particular tension in the concept of a Christian "holy war", since Christianity is unique in unequivocally condeming violence in its scripture. You might want to check out Pope Urban II's famous speech at the Council fo Clermont. It's cleverly structured, in a way that I think is designed to adress that tension. He starts off by lamenting the violence in the West, feudal lords fighting each other, knights being unchivalrous, breaking the Peace of God etc. Then he speaks of what is happening to the Christian populations in the East, talking of desecrations, torture, executions and rape. And he concludes with his call for Crusade, saying "Let those who have been accustomed unjustly to wage private warfare against the faithful now go against the infidels and end with victory this war which should have been begun long ago. Let those who for a long time have been robbers now become knights. Let those who have been fighting against their brothers and relatives now fight in a proper way against the barbarians. Let those who have been serving as mercenaries for small pay now obtain the eternal reward. Let those who have been wearing themselves out in both body and soul now work for a double honor." Basically "since you're being violent sinners anyway, you might as well direct that violence against the enemies of Christianity".

What is less justifiable of course is the massacre upon the taking of Jerusalem. The sack of Constantinople is a bit of a different story though, like I mentioned in the comments on a Byzantine episode and others commented here, that didn't just happen out of nowhere. Overall, while it's perfectly true to say that atrocities were committed during the Crusades, you can't consider the Crusades themselves to be any less justified than any other war (and I would say that they were indeed quite a bit more justified, and most definitely not "unprovoked"). The only way to condemn them is within the radical pacifist philosophy of original Christianity, which makes all violence unacceptable, regardless whether or not it is provoked. But that view had of course been seriously tempered by pragmatic considerations already long before the Crusades, as you mentioned in this episode.

In reply to by Zarl

Peter Adamson on 2 May 2019


Well made point. But I think there is an implicit assumption behind a lot of what you are saying, namely that any Christian military or political force at the time could take itself to be "provoked" by any attack on a Christian community by members of another religion, anywhere on earth. So for instance the king of France would, by your logic, be provoked into justified warfare by a Seljuk attack on Anatolia. Clearly that seemed reasonable to many medieval observers but I am doubtful whether that really fits the medievals' own supposed definition of just war, which is what we are talking about here (not our own feelings about the crusades). My understanding of that definition is that it means being forced to defend your own territory, like if France itself is attacked. And all I meant (if I am remembering rightly - this was quite a while ago!) is that the Crusades didn't satisfy this criterion. I am more moved by your point that Constantinople was asking for help, since you could argue that coming to the air of an ally should be included within the medieval notion of a just war; of course if this is the primary rationale and justification then the 1204 sack becomes yet more horribly ironic.

In reply to by Peter Adamson

Zarl on 3 May 2019

Crusades and just war theory

You're right about that assumption, but I think it's a valid one. Let's take Isidore of Seville as you quoted :

"A war is just when, by a formal declaration, it is waged in order to regain what has been stolen, or to repel the attack of the enemy."

We can also look at Saint Augustine on the matter:

"Those wars are customarily called just which have for their end the revenging of injuries, when it is necessary by war to constrain a city or a nation which has not wished to punish an evil action committed by its citizens, or to restore that which has been taken unjustly."

Neither actually defines the subject, so I see no reason to assume that this should only be valid for a state. We're talking about a time before the emergence of nation-states in the West, when kingdoms were essentially held together by feudal rules of personal loyalty to the king, who himself took his authority from the Christian God. I'd say that at the time, Christendom would actually have been seen as a far more legitimate group identity, if not even as the only truly legitimate one. My guess is that a medieval Christian would simply have thought it obvious that any principle of self-defence applicable to mere kingdoms would a fortiori be applicable to the far more important and essential community of Christendom. And then there was the Byzantine plea of help on top of that. As for the conditions themselves, I think they apply: restore the Holy Land which had been stolen (yes a while before, but neither Isidore nor Augustine mentions a statute of limitations), and repel the more immediate attack on Byzantine Anatolia.

Though come to think of it Saint Augustine's conditions seem vague enough that they could be taken to justify a lot. I just had the thought that Enrico Dandolo could have argued that the sack of Constantinople was just on Augustinian terms, since it could be portrayed as a revenging of injuries, namely that he was promised payment for toppling the Byzantine usurper and putting the previous murdered emperor's son on the throne, but payment never came. It might also have been seen as a punishment for the evil actions previously committed by Byzantines in the Massacre of the Latins. But you're quite right that this wasn't the view taken at the time at all. In fact I hesitate to even call that whole enterprise a Crusade, given that the entire Crusader army had already been excommunicated before it even reached Constantinople. That was definitely never considered a "just war", more like an attempt from the Venetian doge to recuperate his losing investment through piracy. I'd say that the condemnation from the Pope in this case shows that the Church did have a fairly robust sense of justice in war, one which included proper Crusades but clearly excluded these Venetian shenanigans.

Funnily though, wars waged for purely pecuniary reasons became more accepted in more recent centuries. I'm reminded of that time France invaded Mexico because the Mexican government refused to pay exorbitant reparations for a vandalised French pastry shop. Though to be fair I don't think the famed Pastry War involved any particularly egregious atrocities, at least against humans.

In reply to by Zarl

Peter Adamson on 3 May 2019

More Crusades

So actually this conversation has shown that there is a deeper historical and philosophical question lurking here, along the lines of: when and to what extent did Christians start thinking of Christendom as one "community" that could play the role of something like a state for the purposes of, e.g. just war theory? My suspicion is that the Crusades were indeed an effect of that way of thinking - as you're suggesting - but also a major reason that that way of thinking first emerged, in that the ideology developed to justify these military adventures was to some extent new. Also it is interesting to think about how this forced the medievals to develop multiple allegiances and senses of identity, especially since these could come into conflict, like say if your feudal lord didn't want you to go off on crusade. But we're getting pretty far beyond my competence at this point, I'm afraid, and into the realms of something more like straight history than history of philosophy.

In reply to by Emily

Zarl on 4 May 2019

Damn Christians, always going around getting persecuted

Ah yes, Levantine Christians getting persecuted as karmic punishment for previously... getting persecuted. I guess it's not so much a karmic balance and more like karmic cement shoes.

In reply to by Zarl

Emily on 4 May 2019


Better to feed with fishes than to sleep with them!

As a larger "community," Christians can be viewed as both perpetrators and victims of persecution. It seems to depend on one's perspective. My heart breaks for all people persecuted for their faith. Perhaps if Church leaders had embraced Jesus' "radical pacifist philosophy" instead of yielding to pragmatic concerns, the Levantine Christians would be better off today.

And now back to philosophy ...

In reply to by Emily

Zarl on 5 May 2019

Back to philosophy

Or more likely all Christians would be in the same situation, persecuted in Muslim-dominated lands. Martyrdom only gets you so far, at least the Christian kind.

In reply to by Zarl

Emily on 5 May 2019


Too bad nobody told Jesus that. Maybe he wouldn't have wasted all his time and energy on The Passion.

In all seriousness, and with respect for your deeply held beliefs, martyrdom is the foundation of Christianity and plays a central role in Islam, as well. Perhaps Peter could redirect our discussion toward a philosophy of martyrdom? Or we could look to some of Maritain's writings? Certainly the problem of evil is a universal one, not bounded by geography or religious belief.

In reply to by Peter Adamson

Zarl on 4 May 2019

Christian identity

I think you're right that the Crusades both presupposed but also further reinforced a common Christian sense of identity. These things often work in a feeback loop like that. Or at least a Catholic one, with the Eastern Church it seems to have instead done more to widen the divide. 

That said, Christian group identity didn't necessarily mean obedience to the Pope, so I don't think people necessarily got that much cognitive dissonance from being Christian yet siding with a king over the Pope. Especially once nations started emerging, the best and earliest example being France. There the king developed the idea that his authority came directly from God and was equal to (or even superior to!) that of the Pope. This culminated in the episode when Philip the Fair, in disagreement with the Pope, just had him kidnapped and replaced with a French Pope and then had the whole Papacy moved to Avignon in France. He couldn't have done this without a strong national sentiment backing him up (see the very different outcome of the Investiture Controversy), yet that didn't prevent the French from considering themselves very Christian.

I think that spiritual unity of Western Christendom continued until Protestantism.

BOBBY SHEPARD on 7 June 2020


I love philosophy and I love these podcast I'm listen to well over 250 of them and they help me to understand the world in a way that I never knew possible. That being said I believe there should be some sort of society or committee that puts these historical artifacts into more pragmatic layman's terms that could help one to discover how to live life and learn from the past. Perhaps a more common commentary. If there are any text that exist like that I would be greatly interested.

In reply to by BOBBY SHEPARD

Peter Adamson on 8 June 2020

putting it into practice

Yes you're right that, although this podcast is aimed at a broad audience, it isn't trying to, like, show you what philosophy can do in your life or how to apply it to modern-day practical issues. An example of that kind of thing is the Modern Stoicism project, so you could check that out.

Andrew on 30 May 2024


I have been re-listening to the Aquinas episodes now you have done the Aquinas revival episode, and oh boy how depressingly relevant and accurate the example you use for the unintended moral licence the double effect doctrine could in theory give at minutes 17-18. Some governments would be impressed it seems.

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