Just War Theory and The Russia-Ukraine War

This is a guest post by Professor Jeff McMahan (Oxford University), as part of the Reflections on the Russia-Ukraine War series, organized by Aaron James Wendland. This is an edited version of an article published in Studia Philosophica Estonica. Justice Everywhere will publish edited versions of several of the papers from this special issue over the next few weeks.

There are three wars currently in progress in Ukraine: the war between Russia and Ukraine, the Russian war against Ukraine, and the Ukrainian war against Russia. It is necessary for the purpose of evaluation to make these distinctions, for the first of these wars is, like the Second World War (understood as a war between allied and axis powers), neither just nor unjust. Only a war fought by one or more belligerents against an opponent can be just or unjust. Many or most of what we refer to as wars consist of a just war on one side and an unjust war on the other – or, to be more precise, a war with predominantly just aims on one side and a war with predominantly unjust aims on the other.

There is no credible understanding of a just war according to which the Russian war against Ukraine is a just war. It is a wholly unprovoked war of aggression intended by those who initiated it – primarily Putin – to conquer Ukraine, annex its territory, and assimilate its population. The motives of the war’s planners are doubtless many and various but some stand out as obvious and dominant. One is to expand the Russian empire until it is at least coextensive with its earlier boundaries under the tsars and the post-revolutionary Soviet dictators. Another motivation echoes the American concern about “falling dominoes” as a reason for invading Vietnam. Many of the states that were ruled by Soviet puppet regimes during the Cold War have, since the dissolution of the Soviet Union under Gorbachev, been adopting more and more elements of Western culture, in particular liberalism and democracy. Ukraine was a falling domino that threatened to become a fully independent, economically flourishing democracy in a large border territory that Russia had repeatedly ravaged in the past – a state that would be an example, highly visible to Russians, of an appealing alternative to Putin’s tyrannical kleptocracy.

Ukraine’s war against Russia is, by contrast, a paradigmatically just war of defense against unjust aggression. These judgments, however, leave open a range of highly important moral questions. Is it permissible for Russian soldiers to fight in Ukraine? What are the limits to what it is permissible for Ukrainians to do in defense against Russia? What is it permissible, or required, for those in other countries to do? Is it, for example, permissible for other countries to impose economic sanctions on Russia even though doing so will harm civilians? Should other countries provide weaponry to Ukraine? And, if so, are there limits to the types of weaponry it is permissible to provide? Is it, for example, permissible to supply Ukraine with cluster munitions? Finally, might it ever be permissible to intentionally kill Russian civilians if this were necessary to prevent the killing of an even greater number of Ukrainians?

Permissible Targets in War

I begin with the first of these questions, even though it is perhaps of no practical interest to readers outside Russia, and it seems unlikely that there will be many, if any, readers of this article inside Russia. Nevertheless, it is at least of theoretical interest that, according to traditional just war theory, Ukrainian soldiers are legitimate targets for Russian soldiers, who act permissibly in attacking and killing them. In this respect, the traditional moral theory coincides with the law of armed conflict, according to which it is not illegal for combatants to fight in a war that is illegal. The view that it is morally permissible for Russian soldiers to kill Ukrainian soldiers is an implication of the traditional doctrine known as the “moral equality of combatants,” which asserts that all combatants in a war have the same rights and duties and that none do wrong unless they violate the rules governing the conduct of war. It makes no difference, morally, whether the war in which they are fighting, or the goals for which they are fighting, are just or unjust.

But what can possibly be said in support of the claim that Russian soldiers are morally permitted to go in tanks where Ukrainians have been living in peace, posing no threat to any Russian or anyone else, and begin to kill members of the Ukrainian military? None of those members had done anything to make him- or herself morally liable to be attacked or killed. Of course, after the Russians invaded, threatening the freedom, well-being, and lives of millions of morally innocent and unthreatening people, Ukrainian soldiers did begin to threaten the lives of Russian soldiers. Yet a substantial proportion of these Ukrainians were, when the invasion began, ordinary civilians who joined the military in response to it and began to threaten Russian soldiers only to defend their fellow Ukrainians. That they then threatened the Russian soldiers does not mean that the Russians had a right to self-defense in the circumstances. They forfeited that right, along with the right not to be attacked, by threatening the lives and well-being of innocent Ukrainians.

As an analogy, imagine that a man carrying a loaded gun enters the house of a family who are strangers to him with the intention of holding the residents captive and perhaps killing some of them. One family member gets a weapon to defend himself and the others. It would be preposterous to suppose that his preparation to defend the innocent people in the house makes it permissible for the intruder to kill him in self-defense.

According to what has come to be known as “revisionist just war theory,” the moral equality of combatants is false. Ukrainian soldiers are not legitimate targets, even if they threaten the lives of Russian soldiers. They are innocent people in the relevant sense, in that they have done nothing to make themselves morally liable to attack. All violent action by Russian soldiers against Ukrainians, civilian or military, is morally wrong – unless, perhaps, it is intended to prevent Ukrainian soldiers from acting in a way that would itself be wrong, such launching a missile against a civilian target in Russia. It does not follow, however, that all Russian soldiers are murderers or attempted murderers – though some of them are. Many of them have been deceived by government propaganda or act under duress, or both. These conditions do not affect the objective wrongness of their action but they do mitigate the soldiers’ culpability.

One interesting though largely theoretical question concerns the comparative wrongness of attacking Ukrainian soldiers and attacking Ukrainian civilians. Suppose that a Russian soldier intentionally kills a Ukrainian soldier, knowing that in doing so he will also kill a Ukrainian civilian as a side effect. Most people believe that, even if killing the soldier is morally wrong, killing the civilian is even more seriously wrong. But most people also believe that it is in general more seriously wrong to kill a person as an intended means than to kill that person as an unintended side effect. If this is true, then, since neither the soldier nor the civilian is liable to any harm at all, intentionally killing the soldier should be more seriously wrong than killing the civilian as a foreseen side effect. Also, as Helen Frowe has noted, killing the soldier increases, though perhaps not significantly, the probability that Russia will succeed in achieving its unjust aims. The greater wrongness of killing the soldier is, however, compatible with the Russian soldier’s being more culpable for the killing of the civilian, as he presumably believes, in accordance with traditional just war theory, that his killing Ukrainian soldiers is not morally objectionable in any way.

The Requirements of Necessity and Proportionality

According to both traditional and revisionist just war theory, it is presumptively permissible for Ukrainian soldiers to engage in violence in the collective defense of all those threatened by Russian aggression. There are, however, conditions in which the resort to or continuation of a war in support of just, defensive aims is not permissible. These conditions are identified by the just war principles of necessity and proportionality.

For a war or its continuation to be necessary in the relevant sense, it must be the morally best means of achieving just aims, taking into account both the probability of success and the probable bad effects, both intended and unintended. The just aim of the Ukrainians is to avoid subjugation by Russia. And it seems clear that when Russian tanks entered Ukraine and sought to encircle Kyiv, there was no alternative at all to armed resistance. It is conceivable that the Ukrainians could ultimately have maintained their political independence by engaging in mass nonviolent resistance, but that would have required years of preparation and training of the civilian population and thus was not an option when the tanks and ground forces arrived.

Unlike the requirement of necessity, the requirement of proportionality does not compare a war or its continuation with alternative courses of action. It instead assesses whether the possible bad effects of a war, taking probabilities into account, would outweigh, or be excessive in relation to, the achievement of the just aims, again discounted for probabilities. Not all bad effects, though, count in determining whether a war or its continuation would be proportionate. Harms inflicted on those who are morally liable to those harms are bad effects but do not count in the assessment of proportionality. Assuming that each Russian soldier fighting in Ukraine is morally liable to be killed by Ukrainian forces, the Ukrainian war against Russia cannot be disproportionate because of the number of casualties among Russian soldiers.

One might wonder whether a war with a just cause could be disproportionate because of the harms that would be inflicted by the other, unjust side on those who are fighting for the just cause and the civilians in whose defense they would be fighting. In some cases, the prospect of such harms do make it disproportionate for the governing authorities to initiate or prolong a war with a just cause, or for other states to enable the government to do this. In a recent interview, Noam Chomsky has suggested that this is true of Ukraine’s war against Russia. The interviewer observes that the only public debate that people are taking seriously is, how much in arms should we give them? And should we simply give them arms? Or should we intervene militarily? And that is the debate. But a more rational way of looking at this, as you say, would be to think about how to prevent Ukrainians from dying in this horrible war.

Chomsky responds by saying that “I would agree except for the word ‘rational.’ It’s the more humane way” and then goes on to explain, in effect, that rationality is distinct from morality. His view is that the risks inherent in the Ukrainian war against Russia are, to use my term, disproportionate, so that the just cause of preserving Ukraine’s political independence and territorial integrity must be abandoned in favor of a “diplomatic settlement” that ends the war “without destroying Ukraine and going on the destroy the world.” The settlement will involve “neutralization of Ukraine, some kind of accommodation for the Donbas region, with a high level of autonomy, maybe within some federal structure in Ukraine, and recognizing that, like it or not, Crimea is not on the table.” Chomsky’s view, in short, is that Ukraine must make concessions to Russian aggression, allowing Russia to achieve some of its unjust aims. And at least part of the reason for this is that the potential harms to Ukrainians (“destroying Ukraine”) are too great for the continuation of the war to be morally justified.

Although I have the most profound respect for Chomsky, I disagree with him here. To the extent that his concern is, as the interviewer suggests, “to prevent Ukrainians from dying,” my view is that it is the Ukrainians’ right to decide whether they would rather endure the risks of continued war or accept the certainty of subjugation to Russia. In an editorial written in September 2023, Thomas Friedman reported, after visiting Ukraine, that “nearly every Ukrainian I spoke to in Kyiv was at once exhausted by the war and passionately determined to recover every inch of their Russian-occupied territory.” (Admittedly, those in Kyiv may have a different view from those in the areas in which most of the fighting is taking place.)

A similar point applies, with certain grave reservations, to the controversy that arose about supplying Ukrainian forces with cluster munitions. These weapons can be effective against concentrations of troops, even in established defensive positions, but tend to leave substantial amounts of unexploded ordnance lying on the ground that may kill civilians, particularly children, even years later. For this reason, some states have agreed to legal bans against their use. Yet, because the ground war against Russia is being fought entirely on Ukrainian territory, those who would be exposed to the later risks from the present use of cluster munitions are the Ukrainians themselves; and it can thus be argued that they should be the ones to decide whether the military benefits from the use of these weapons are sufficient to outweigh the future risks (which they could minimize through concerted intra- and post-war efforts to locate and remove the undetonated explosives).

Chomsky is certainly right, though, to suppose that the larger risks to the world could render the Ukrainian war of defense disproportionate. His principal concern is with the risk of escalation to nuclear war. That risk is of course ever-present in this war but seemed greater at the time he was interviewed, in April of 2022, when there was uncertainty about what Russia’s response might be if Western states were to supply Ukraine with various forms of advanced weaponry – particularly following Putin’s several explicit threats to use nuclear weapons. It now seems, however, that the continued provision of the same types of weapons that have already been provided and been militarily effective involves little risk of provoking the Russians to use nuclear weapons.

The same cannot be said, however, about direct military intervention in support of Ukraine, particularly by states that are members of NATO. That seems clearly ruled out as disproportionate because of the risk of escalation. Although some just war theorists are reluctant or unwilling to conceded this, unjust aggressors can render what would otherwise be permissible defensive action disproportionate by credibly threatening sufficiently serious harms in response to such action. In the present case, if there were a sufficiently high probability that Putin would use nuclear weapons against NATO forces were they to intervene in support of Ukraine, and if, whatever intentions they might have in advance, NATO powers might then use nuclear weapons in retaliation, I think Putin would have made it disproportionate and therefore impermissible for NATO forces to intervene militarily in Ukraine.

The deterrence of conventional intervention has, indeed, been one of the two most important uses of nuclear weapons by their possessors. In addition to deterring nuclear attacks, nuclear weapons have consistently been used, usually without any need for explicit threats, to deter third party intervention in support of the victims of conventional attacks (usually though not always unjustified) by a nuclear-armed state.

Another ground for concern that the Ukrainians’ pursuit of their just aims by means of war might be, or have been, disproportionate is that, at certain times, the war between the two states has threatened to cause mass starvation in certain areas of the world by preventing the shipment of grains and other foods from both Ukraine and Russia to those who had previously relied on those supplies for their survival. At present this does not seem to be a reason for judging the Ukrainian war against Russia to be disproportionate but that could change. It is conceivable that the Russians could use the threat of starving entire populations as a means of rendering the continuation of the war by Ukraine disproportionate. One must, however, set against this one good effect that the Ukrainian war against Russia is having internationally, and that is to strengthen the deterrence of other potential aggressors. As Paul Krugman notes in a recent editorial, “Russia’s failures in Ukraine have surely reduced the chances that China will invade Taiwan.” In this way, as many have observed, the sacrifices by the Ukrainians have been of great service to all peoples at risk of unjust attack.

Liability to Harm in War

Earlier there was reason to hope that the coordinated implementation of economic sanctions against Russia by many of the world’s states could make a significant contribution to the effort to prevent Russia from succeeding in achieving its aims in Ukraine. If Putin and his fellow oligarchs could have been persuaded that among the costs to them of continuing the war would be the substantial weakening of the Russian economy and rising discontent among the citizens, they might have been motivated to find a way of saving face while bringing the war to an end. But there were two main concerns about sanctions at that time.

One was that sanctions would be largely ineffective. That concern seems to have been to a considerable degree warranted. Russia’s efforts to achieve its aims in Ukraine have so far been frustrated by military resistance but it is doubtful that economic sanctions have been a significant supplement to military action. There is, indeed, some reason to believe that they may have been counterproductive overall, in much the way that Russia’s attacks on power stations in Ukrainian cities have been. As both the Germans and the British learned, or should have learned, in the Second World War, efforts to demoralize enemy civilian populations by harming them often have the opposite effect, causing people to hate those who harm them even more, thus reinforcing their support for their own government.

The other concern that the broad-ranging sanctions raised was that they might constitute terrorism insofar as they have been intended to impose hardship on the civilian population as a means of motivating ordinary citizens to exert pressure on the Russian government to end the war. If terrorism consists in intentionally and harmfully using innocent people as a means of influencing the action of others, then the imposition of sanctions with the intention of inflicting burdens on ordinary civilians may constitute terrorism, albeit of a quite minor sort.

This reason for concern about the permissibility of sanctions seems to me less serious than the doubts about the efficacy of sanctions. For most Russians, the effects of the sanctions have been comparatively minor and have not violated their rights. Citizens in Moscow, for example, have never had a moral right to be able to eat McDonald’s hamburgers. Nor have most Russian citizens been entirely morally innocent. When the leader for whose being in power they bear some responsibility, however small, initiates an unjust war against a neighboring population – and particularly one that their state has a long history of oppressing – they have a responsibility, and perhaps a duty, to register some form of objection, even if only clandestinely, given the risks involved in open protest. It is true, of course, that no one Russian civilian on his or her own could have prevented the initiation or continuation of the Russian war against Ukraine; but a certain number of them acting together could have. That most Russians have failed to fulfill their responsibility to oppose the war – and indeed have continued to support the war – may make them morally liable to suffer the comparatively minor harms that have been caused by the sanctions.

The theoretical point behind this perhaps surprising claim about civilian liability is that liability to harm is essentially comparative. Liability arises when harm is unavoidable but distributable – that is, when someone must be harmed but who it will be is a matter of choice. A person is morally liable to be harmed when it is comparatively more just that he be harmed – typically because he is responsible for the fact that harm is unavoidable – than that anyone else be harmed, taking the magnitudes of the different harms into account. Liability, in short, is a matter of comparative justice in the distribution of harm when harm is unavoidable.

In this case, Russia’s invasion of Ukraine made great harm unavoidable. If sanctions could be effective – and, as I noted, it was reasonable earlier to believe that they could be – then the choice that potential imposers of sanctions had was between a high probability of minor harms to a large number of Russian civilians and a reduction in the probability that some smaller number of morally innocent Ukrainians (civilians and soldiers) would be killed by Russian soldiers. And whether and to what extent an individual is morally liable to be harmed on a particular occasion is a function not only of the degree to which that individual is causally and morally responsible for a threatened unjust harm but also on the magnitude of the unjust harm that is likely to be averted by harming that individual.

Common views about the ways in which harms and benefits aggregate might alone provide a justification for choosing to impose sanctions in these conditions. This justification would not be based on considerations of liability but would instead be a “lesser-evil justification.” Claims about aggregation are often illustrated by such choices as that, for example, between preventing a large number of people from experiencing a minor headache and preventing a small number of people from dying. It may be claimed that there is no number of people whose headaches it would be better to prevent rather than the saving of the lives of just a few people. This is of course a claim about preventing harms rather than causing them; but if it were true that, whatever one did, one would either cause each of a very large number of people to have a minor headache or cause only one person to die, it seems intuitively that it would be better to cause the headaches no matter how many people would suffer them. And a similar claim might be made about the choice between causing a large number of Russian civilians to suffer minor harms from sanction and allowing a much smaller number of Ukrainian civilians to be killed.

But once we factor in the fact that most Russian civilians bear some very small degree of responsibility for the fact that some harm is unavoidable in the circumstances, while Ukrainian civilians (and soldiers) bear none at all, it seems that there is not only a lesser-evil justification but also a liability justification for inflicting small harms on a very large number of Russian civilians as a means of reducing the probability that a much smaller number of Ukrainians will be killed – as well as reducing the probability that many more Ukrainians will have to live under Russian domination. (There has always, as I noted, been uncertainty about the effectiveness of economic sanctions. But insofar as some people must bear the costs of that uncertainty, they should, as a matter of comparative justice, be Russian civilians rather than Ukrainians, other things being equal.)

There are, of course, many courageous and morally admirable Russians who have openly protested against their country’s war, many of whom have suffered serious harms through imprisonment and other punishments at the hands of their government. Some of these individuals have also experienced additional though lesser harms as a side effect of the sanctions, which only compounds the injustices they have suffered. One hopes that they nevertheless approve of the sanctions, at least to the extent that they have been effective, and excuse those who have imposed them as both groups share the same cause of ending Russia’s unjust and cruel war.

Terrorism in the Russia-Ukraine War

I will conclude with a more serious concern about terrorism. The Russian war has been consistently terrorist, with repeated bombings of residential buildings, the rape, torture, and killing of civilian detainees, the intentional destruction of power stations to deprive ordinary Ukrainians of heat in winter, cooked food, and clean water, and so on. Although Ukraine has been capable of terrorist reprisals, it has, at least until recently, largely refrained from exercising that capacity. Around mid-2023, there was a successful drone strike on the Kremlin, though the explosion was small and the damage minimal. As this was a strike on the political source of the Russian war, it cannot be considered a terrorist act. (Nor, I think, was the attempt to kill Aleksandr Dugin, which inadvertently killed his daughter Daria Dugina instead, an act of terrorism by whoever conducted it. Although they were civilians, both Dugin and his daughter were highly influential and effective propagandists for the Russian war who were therefore both causally and morally responsible to a significant degree – perhaps to a higher degree than most Russian combatants – for the threats to innocent people and just institutions in Ukraine.) There were also, however, other small-scale drone strikes in Moscow that may have warranted Putin’s description of them as “terrorist.”

More recently there was a Ukrainian missile attack on the Russian city of Belgorod, near the border with Ukraine, that killed at least 22 people and injured many more. This occurred the day after Russian fired more than 150 missiles at various Ukrainian cities, damaging or destroying factories, hospitals, and schools. And just prior to the attack on Belgorod, President Zelensky announced that Ukraine would “work toward pushing the war back” to “where it came from – home to Russia.” These facts support the suspicion that the Ukrainian attack on the city was a reprisal for the attacks on Ukrainian cities – that is, a terrorist reprisal to terrorist attacks. Although a Ukrainian official claimed that only military facilities in Belgorod had been targeted, the official did not offer in any evidence in support of this claim. One hopes that the official’s statement is true, but in the absence of detailed evidence about the alleged targets and what was actually destroyed, skepticism cannot be lightly dismissed.

Even if many Russian civilians are, in the circumstances, liable to small harms from economic sanctions, they do not bear sufficient causal or moral responsibility for their government’s unjust war to make them liable to be killed. If, therefore, the attack on Belgorod was indeed intended as a reprisal-in-kind for the Russian attacks on Ukrainian cities, it was a terrorist act for which there was no moral justification. Indeed, in addition to being immoral, the attack on Belgorod may ultimately be counterproductive or self-defeating, both because it may tarnish Ukraine’s international reputation as the innocent victim of aggression and terrorism that nevertheless abides by the moral and legal rules of war, and because it may alienate the sympathies of some of the Russians who at least until now have supported Ukraine in whatever ways they could. These people may be rightly reluctant to express or provide support for a military that intentionally kills Russian civilians.

There are, I concede, difficult theoretical issues here. Suppose, however unrealistically, that it were true that Ukraine’s intentionally killing 500 Russian civilians by means of drone attacks on Moscow would cause the Russian government to end the war in Ukraine and withdraw from all Ukrainian territories except Crimea. And suppose that all of the Russian civilians who would be killed would be adults who voted for Putin and have consistently supported the Russian war. Killing these people would prevent not only the killing of a far greater number of Ukrainians, including children, but also the subjugation of Ukraine by Russia. In these conditions, some killings are, from the Ukrainian perspective, inevitable or unavoidable. Assuming, as I claimed earlier, that liability is comparative, it might be argued that it would be more just that a smaller number of civilians who bear some tiny responsibility for the fact that some killings are unavoidable should be killed than that a much greater number of civilians who bear no responsibility for this fact should be killed. If so, it seems that there should be a liability justification for intentionally killing the Russian civilians as a means of ending the war – that is, that the Russian civilians are, in the circumstances, morally liable to be killed. And if the Russian civilians are liable to be killed, killing them would not be an instance of terrorism, since terrorism is the intentional infliction of certain harms on people who are not liable to those harms as a means of manipulating the action of others.

Even if one’s intuition is that it would be permissible to kill the 500 Russian civilians, one need not accept that they are liable to be killed; for there might instead be a different form of justification for killing them. If killing them would avert vastly greater harms to wholly innocent Ukrainians, it might be justified as the lesser evil. The Russians would not be liable to be killed; hence killing them would wrong them and infringe each one’s right not to be killed. But their rights would be overridden by the need to defend the similar rights of a far greater number of Ukrainians. Killing them would indeed constitute terrorism, but terrorism that could be morally justified as the lesser evil.

There are thus three possibilities: (i) that the Russian civilians are liable to be killed so that killing them would not be terrorism, (ii) that killing them would be justified terrorism, and (iii) that killing them would be unjustified terrorism. The case for the claim that they are liable to be killed is based on the view that liability is a matter of comparative justice. Suppose, as I claimed earlier, that a large number of Russian civilians could be liable to suffer the comparatively minor harms inflicted by sanctions because they bear some very small degree of responsibility for the fact that harm is unavoidable, whereas Ukrainians bear none at all. In that case, Russian civilians could in principle be liable to greater harms, and perhaps even be liable to be killed, if killing them were the only way to reduce the risk of a vastly greater number of Ukrainians being killed.

One might object to this argument that, if the degree of person’s responsibility for a threatened harm is very slight, then the harm to which that person can be liable on the basis of that responsibility must be slight as well, as in the case of the harms inflicted on Russian civilians by economic sanctions. This objection is challenged, however, by a common – though certainly not universal – intuition about an example that I refer to as ‘The Conscientious Driver’:

A person has chosen to take a drive in her car purely for pleasure. She is driving carefully and alertly and has kept her car well maintained. Still, despite her precautions, her car malfunctions and veers uncontrollably off the road and, unless it is stopped, will kill a man having a picnic in the grass well away from the road. Some soldiers in a convoy are parked nearby and one of them can use a weapon he has to blow up the car, thereby saving the picnicker though killing the driver.

My view is that, because the driver chose to engage in a morally optional activity that she knew would expose each of a large number of other people to a negligible risk of great harm, she is responsible – though not culpable – for the fact that either she or the picnicker will be killed. Because of that, considerations of justice dictate that, if all other considerations are equal, she should be the one to be killed – that is, that she is liable to be killed to prevent her from killing the picnicker. But, if this is right, then a person can be morally liable to be killed on the basis of a minimal degree of responsibility for a threatened harm, though the harm must be at least as serious as that of being killed. Hence some Russian civilians who bear only a minimal degree of responsibility for the war might be morally liable to be killed if that were necessary to prevent the killing of a greater number of wholly non-responsible Ukrainian civilians.

There are, however, other differences that are probably morally significant between the case of the conscientious driver and that of the Russian civilians, such as that the killing of the driver would not use her as a means of saving the picnicker, though killing the Russian civilians would harmfully use them as a means of saving Ukrainians. I cannot, however, pursue these other differences here and must therefore leave it an open question which of the three views I noted above is the correct view about the hypothetical example in which killing Russian civilians would end the war in Ukraine. I do think that this example is of more than merely theoretical interest and hope that these brief and inconclusive thoughts about it will stimulate further discussion in the future.

Conclusion

In summary, I have argued that, because the Russian aim of conquering Ukraine and incorporating it into Russia is an unjust aim, virtually any use of force by the Russian military is impermissible. Ukraine and its supporters are, however, also bound by moral constraints, such as proportionality, that Russia can exploit to its advantage, perhaps making it impermissible for Ukraine to fully achieve all of its just aims. There are also moral constraints on what Ukraine may do that would be harmful to Russian civilians. I have argued, however, that not all harms that might be intentionally inflicted on at least some Russian civilians would constitute terrorism. But, for both moral and prudential reasons, Ukraine should not retaliate against Russian terrorist attacks by itself attacking civilian targets in Russia – except, perhaps, in the direst conditions in which there would clearly be either a lesser-evil justification or a liability-based justification for doing so.

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Jeff McMahan is the Sekyra and White’s Professor of Moral Philosophy at the University of Oxford. He is the author of several books, including The Ethics of Killing and Killing in War, and he is currently editing a set of books for Oxford University Press on the legacy of his greatest teacher and good friend: Derek Parfit.