Michael Walzer and Jeff McMahan have each recently written about the war on ISIS. The authors are the leading philosophers of war. Walzer’s book Just and Unjust Wars set the standard for the discipline, and McMahan’s work —including his book Killing in War– innovates just war theory, in part by challenging some of Walzer’s theses. So it is notable that in their recent pieces Walzer is skeptical of the war, while McMahan endorses it. After analyzing and comparing both pieces, this note puts forth a fresh proposal.

Writing for Dissent, Walzer distinguishes between just war and just policing. Just war principles apply to traditional wars; just policing principles apply to terrorists at home. As to just war, he concedes that the Western nations have a just cause against ISIS. However, he says that this war is unjust because it cannot possibly succeed. Having a reasonable probability of victory is, for Walzer, one condition of the justice of war. But not just any victory will suffice; the just war must be aimed to a just end. Walzer believes that the West’s present strategy cannot succeed, whether limited to airstrikes or expanded to include ground forces. He points out that the current air strikes are minimally coordinated and the various factions are enemies of one another. Therefore, even if the current coalition commits ground troops and defeats ISIS, there are no feasible solutions that lead to a just end. He writes

Suppose that ISIS is defeated, what then? Are the Sunni Muslims in the caliphate to be returned to Shiite rule in Iraq and Alawite rule in Syria? That isn’t a prospect likely to inspire any of our Sunni allies. A just war must aim at a just ending, but this war is being fought without any likely end and without any vision of what a morally just end would look like. Without those two, I find it hard to defend the current air war—which may well kill more innocent people than ISIS fighters and produce more ISIS fighters than it kills.

Walzer then turns to the war on terror here in the United States. He argues that the “war” here is a question of policing. How to do so justly is a matter of widespread and impassioned debated today. Walzer correctly insists that just policing must be kept apart from just war, especially because the former ought to be conducted within stricter precepts of the rule of law and civil liberties (for example, the police cannot use the principle of proportionality in the same way it is used by war commanders).

Walzer’s distinction between just war and just policing is important, and his suggestions here make sense. As I have argued elsewhere, off the battlefield a state may not declare war on terrorists—or anyone else—and simply start shooting these persons on sight, but on the battlefield of war, they can.

However, I am not persuaded by Walzer’s claim that the war against ISIS is unjust. He concedes that this war would be just if it could be won, so he accepts the justice in principle. But he believes Coalition forces—and here, I mean any suitable military alliance—cannot in fact achieve a just end. Walzer does conceive of one just outcome of the war—wherein the Sunnis or Kurds achieve autonomy or independence. But, he writes, “it is very hard to figure out how to organize a war of that sort.”

I agree the Coalition has a just cause, but I am more optimistic about the ability to implement a just end. Even Walzer concedes that the elimination of ISIS and the establishment of political structures that would prevent its resurgence, would constitute a just end. And although Walzer is correct this will be extremely difficult to implement, it is not beyond the realm of possibility. This future would require leaders with the imagination and resolve to prosecute an ethical and legal war and bring about the just end. It might require the Coalition to occupy Syria and Iraq longer than the public would currently stomach, and it certainly requires the cooperation of indigenous forces and populations. But none of this is impossible or unachievable. Victorious just warriors have faced such difficulties before, and have managed to resolve them.

In the Washington Post, Jeff McMahan argues that the war against ISIS is justified, not as a reaction against ISIS’s crimes in Paris and elsewhere, but because it is necessary to save the people in Syria and Iraq who are being tormented by ISIS. McMahan’s position is that this war can be justified as humanitarian intervention. McMahan’s describes some of the horrors faced by those subject to ISIS rule:

Entire cities. . . . have been under the Islamic State’s control for more than a year. In areas where they rule, beheadings in public spaces are daily occurrences. Execution by the sword is the penalty for offenses including apostasy, blasphemy, and homosexuality. According to many members of the Islamic State, indications of apostasy among Muslims include shaving, voting, selling alcohol and being Shiite rather than Sunni. Adultery is punished by stoning, theft by mutilation, consumption of alcohol by 80 lashes, and other offenses by crucifixion. Trials, to the extent that they occur at all, are conducted by fanatical clerics and may last only a few minutes. Hostages are routinely tortured. Captured infidel women, such as those from the ethnic and religious Yazidi minority in Iraq, are sold as sexual slaves whose treatment is governed by a set of arbitrary religious ordinances which state, for example, the conditions in which it is permissible for a man to rape a pre-pubescent girl. Sunni males as young as seven are being conscripted into the fighting units, and are taught techniques of beheading by practicing on captured soldiers from the Syrian army. The least intelligent are chosen for suicide missions.

And there is no doubt, McMahan reminds us, as to ISIS’s long-term goals:

With the attacks in Paris and the destruction of the Russian passenger plane, the Islamic State has brought its war to areas beyond the Middle East. This was predictable, as the aim of its war is to expand the newly-declared caliphate until it encompasses the world, exterminating apostates and converting or subjugating infidels in the process. There will, therefore, be more such massacres.

Surely if any cause is just, the fight against this sort of evil qualifies. McMahan makes two further points. First, he views a ground war as necessary to defeat ISIS. He cautions that ground troops must be indigenous, or ISIS could use Western imperialism as a continuing pretext for its crimes and further recruitment. Second, the West must open its doors to those who are fleeing from ISIS, thus combining military efforts abroad with generosity at home.

Both McMahan and Walzer agree that the coalition has a just cause to wage war against ISIS. McMahan, however, does not address the issue of feasibility that worries Walzer. One reading of McMahan’s piece is that the just end is possible, because the goal is simply to rescue ISIS' victims. This would present the primary divergence in the two philosopher’s views. For Walzer, the war cannot achieve its end because the end is to stabilize the region, and this is unfeasible. For McMahan, the war can achieve its end because the end is to stop ongoing genocide, even if post bellum arrangements remain uncertain. And perhaps this demonstrates a weakness of McMahan’s thesis. If the coalition stops the genocide but fails to eradicate ISIS because it cannot implement the necessary political arrangements, then there will be future massacres. Indeed, given ISIS's goals, even McMahan grimly predicts “more massacres” will periodically occur. Walzer’s points about winning and the importance of realizing a just end to the war remain valid.

As to the just cause of war, I agree with McMahan that ISIS’s crimes in the Middle East are enough to justify intervention. However, I am unconvinced by McMahan’s offhanded dismissal of the West’s right to respond based on ISIS attacks in Paris and elsewhere. In my judgment, the justice of the war on ISIS is overdetermined. It is justified as both humanitarian intervention and as a war of self-defense. But the rationale for war extends only to the battlefield in the Middle East. As Walzer underscores, if we are to be faithful to American principles, terrorists at home must be addressed with policing tools.

McMahan is likely correct that ground forces should be indigenous. However, his reason is that the presence of Western troops facilitates ISIS propaganda. In contrast, I believe that indigenous troops are preferable because their region, their future, their land, and their rights are at stake. And for this reason, local populations should claim the responsibility and bear a good part of the cost of destroying this scourge and recovering their societies. To the extent indigenous troops are unable to ouster ISIS, then Western troop intervention may be justifiable on humanitarian grounds. Stopping genocide is the highest priority; the nationality of troops is of lesser importance.

I likewise agree with McMahan’s position on refugees, though I would add one friendly amendment. Liberal democracies should be generous with immigrants –not just with refugees, by the way— but should be able to screen out criminals and terrorists. And this is easier said than done. Germany has been unable to weed out bad actors for a number of complex historical reasons. The United States has similar problems, but because of simple inefficiency.

I would propose the following general just-war framework for the war on ISIS. This framework combines the views of both authors and incorporates a few of my own.