The past month’s Syria debates have focused on issues of policy and morality, with international law a distant third. The nearly universal agreement that the moral issues matter is perhaps somewhat surprising in a world of Realpolitik. I find it heartening. (The lack of interest in international law is less heartening, but that is not my topic.) But how should we think about the moral issues?

Moral revulsion at the chemical attacks is straightforward; what is less straightforward is the morality of a military response. The dominant analytical framework is the so-called just war theory, developed over centuries by philosophers and jurists, beginning with St. Augustine and St. Thomas Aquinas. Despite its religious roots, the six standard just-war criteria accepted today are secular. The war must be waged for a just cause by a legitimate authority acting with right intention. It must be the last resort and have reasonable prospects of success. And the harm the war can be expected to inflict must not be out of proportion to the importance of its goals.

According to the just war theory, failure of any of these criteria makes the use of force unjust. Even so, it seems to me that for years academic debates focused almost exclusively on the issue of just cause. The same is true outside the academy: the just-cause issue dominated the debate a decade ago about whether “anticipatory self-defense” in Iraq was a just cause, just as the Kosovo debate five years earlier centered on whether humanitarianism is a just cause. Neither question was definitively answered, but these profound debates had the effect of narrowing arguments about just war to the single issue of just cause.

Of course, political debates also focused on whether unilateral action without UN authorization violates the requirement of legitimate authority; but the U.S. successfully finessed the question by forming coalitions rather than going it alone. The question certainly matters in the Syria debates, and I’ll get to it. First, though, let’s follow up on whether humanitarian interventions satisfy the criterion of just cause.

Interestingly, theorists had anticipated the post-Kosovo debate about humanitarian intervention by twenty years. Modern just war theory came to life when political theorist Michael Walzer published his masterpiece, Just and Unjust Wars, in 1977. (It is now in its fifth edition.) Walzer anchored the concept of just cause in individual human rights, with a surprising communitarian twist: he argued that one of the key individual human rights is the right to live in a self-determining political community of one’s own—in other words, in a sovereign state. The fundamental human rights violation is therefore aggression against another political community’s sovereignty. Walzer’s view echoed the judgment of the Nuremberg Tribunal: “To initiate a war of aggression … is not only an international crime; it is the supreme international crime.” The primacy of state sovereignty  became the key idea of the UN Charter, which proclaims that “the Organization is based on the principle of the sovereign equality of all its Members” (article 2(1)). The Charter therefore bans the threat or use of force against the territorial integrity or political independence of other states. The sole exceptions are actions authorized by the Security Council and actions taken in self-defense—and self-defense means the defense of sovereignty, not of individual human rights.

The trouble with giving state sovereignty such an exalted position is that sometimes the state itself is the human rights problem. Walzer’s early critics tellingly objected that if a state tyrannically violates the basic human rights of its own citizens on a large scale, it hardly makes sense to defend its sovereignty on human rights grounds. Sovereignty should not set an absolute barrier to humanitarian intervention. (Walzer himself would permit humanitarian intervention, but only in very restricted circumstances.)

Remarkably, this did not remain a theoretician’s debate. In a famous 1999 speech to the UN’s General Assembly, Secretary General Kofi Annan argued that state sovereignty has been redefined so that today “the State is … the servant of its people, and not vice versa.” Ergo, no state has a sovereign right to attack its own people. It seems to follow that a humanitarian intervention to stop a massacre would not violate state sovereignty; and the Responsibility to Protect doctrine (abbreviated R2P), developed a few years later, would allow humanitarian military intervention as a last resort against atrocity.

Annan himself pointedly refrained from drawing that conclusion, because he worried about “undermining the imperfect, yet resilient, security system created after the Second World War, and of setting dangerous precedents for future interventions without a clear criterion to decide who might invoke these precedents, and in what circumstances.” Annan forcefully presented both sides of the argument and let the world draw its own conclusions about humanitarian interventions.

In 2006 the Security Council endorsed R2P, but reserved to itself the authority to launch humanitarian interventions. In its 2011 Libya resolution, the Council invoked R2P when it authorized “Member States acting nationally or through regional organizations or arrangements” to use force to protect Libyan civilians. Apparently, the humanitarian intervention side of the argument had prevailed.

But the Libyan war enraged Russia, which regarded its evolution from humanitarianism to regime change as a bait and switch. This is one reason that Russia opposes Security Council action in Syria. It is hard to imagine sovereignty-sensitive Russia and China permitting any future R2P operations.

Remarkably, the Syria debates have added some new wrinkles to these old debates, and may presage a revision in just war theory itself.

1. By far the most important is that just cause no longer dominates the debate, at least in this country. Assad’s actions have been so monstrous that few Americans think that just cause is even an issue. Of course stopping the slaughter and the chemical attacks is just.

Instead, the key issue has been reasonable likelihood of success. The president’s key limitations—no boots on the ground, no open-ended operations, no prolonged air strikes—simply haven’t seemed capable of accomplishing any humanitarian ends at all. (Satirist Andy Borowitz wrote a mock news story with the headline “Obama Promises Syria Strike Will Have No Objective,” with a quote from the President: “We will simply do something random there for one or two days and then leave.”)

In his Syria speech, the President tried to reassure Americans that this is not so: “This would be a targeted strike to achieve a clear objective:  deterring the use of chemical weapons, and degrading Assad’s capabilities.” The trouble is that deterrence is not a clear objective. Unlike destroying a physical target, it is impossible to know what kind or level of military force would achieve deterrence—or even whether it has been achieved. How do you tell if Assad has been deterred or is merely biding his time? Fortunately, the current diplomatic solution may spare the U.S. from having to find out.

At the time of Kosovo, Tony Blair issued an important pronouncement on humanitarian uses of force. The Blair Doctrine requires “military operations we can sensibly and prudently undertake”—which surely implies that those operations have a reasonable chance of success—as well as being prepared for the long term, which President Obama explicitly ruled out. Five years earlier, President Clinton’s Directive 25 required that “there are clear objectives” and “the means to accomplish the mission are available.” In Syria, President Obama’s strict limits on what he is prepared to do violate all these requirements. The unhappy fact is that we have no reason to suppose that severely limited U.S. strikes would save civilian lives.

2. This jarring discrepancy between humanitarian ends and chosen means suggests an important refinement of the just-cause criterion itself. The idea is simple: a humanitarian intervention must demonstrate a reasonable relationship between means and ends to even become a candidate for counting as just cause. If humanitarianism is the aim, the chosen means must be able to accomplish the humanitarian end. If the means are too limited to accomplish it, the use of force is not truly humanitarian.

3. Very likely it is for this reason that President Obama redefined the goals of his threatened Syria strikes. Although his moral case is still fundamentally humanitarian, the stated purpose of the threatened strikes is not saving lives but upholding and reinforcing the world-wide norm against chemical weapons. Ironically, a unilateral U.S. threat to use force—plainly illegal under the UN Charter—is now justified as law enforcement. (Neither the U.S. nor Russia have mentioned another, lesser, irony: both countries missed the April 2012 deadline for destroying their own chemical weapons, and the U.S. Army has announced that it will not complete the job until 2023. As my colleague David Koplow has shown, both countries are at least technically in breach of the very treaty they have forced Syria to join.)

Perhaps this will be the first step toward a new norm of just cause: war as a form of law enforcement. Early in the Syrian debates, administration figures spoke loosely of “punishing” the Assad regime for using chemical weapons. If they meant retribution, that is clearly illegal and—I have argued—immoral. Criminal leaders can sometimes be punished by international courts like the ICC, but states cannot sentence other states to punishment by war. It would be grotesque if they could. But the administration has made it clear that they are talking about deterrence, not retribution, and that runs less danger of states engaging in vigilante payback. Even so, this rationale poses a clear challenge to the UN Charter’s restriction of just cause to self-defense, or even self-defense fortified with R2P.

4. That takes us back to the question of legitimate authority. If the Security Council won’t act, can a single state unilaterally take on the role of policeman, judge, and executioner? Although President Obama’s Syria speech emphasized that the U.S. “should not be the world’s policeman,” he contradicted himself by also saying “the United States has been the anchor of global security.  This has meant doing more than forging international agreements — it has meant enforcing them.” That, of course, is another way of saying that the United States must be the world’s policeman.

The United States blamed Russia’s power to veto Security Council action for this state of affairs. Earlier this month, UN Ambassador to the United Nations Samantha Power said, “Russia continues to hold the council hostage and shirk its international responsibilities.” She explained that “the system devised in 1945 precisely to deal with threats of this nature did not work as it is supposed to.” Instead, “the system has protected the prerogatives of Russia.” The implication is that when the system fails, the United States is entitled to step in.

Ambassador Power is plainly right about Russian obstructionism, but it is far from clear that the system did not work as it is supposed to. The founders of the UN hoped to avoid the failure of the League of Nations by giving the new organization teeth, underwritten by the great powers. But the great powers drove a Machiavellian bargain: they would not join without a veto power to protect what they saw as their own vital interests. Certainly the United States has used the veto threat on numerous occasions. Arguably, this is exactly the way the system was supposed to work—it was built around the force of the great powers for good or for ill. No doubt this is what Kofi Annan had in mind when he called the UN regime an “imperfect, yet resilient, security system.” The capacity of the great powers for good could never be disentangled from their capacity for ill. The power to enforce came bundled together with the power to obstruct.

This is the system that the United States claims moral authority to bypass. In effect, the United States has appealed to an older, and (some might say) higher law of legitimate authority. At the founding of modern international law, Hugo Grotius argued that every state has legitimate authority to use force in response to “gross violations of the law of nature and of nations.” It appears that the United States has accepted a “Grotian override” of the elaborate Security Council system.

In all these ways, we can see the contours of a new version of just war theory: a version that emphasizes a reasonable relation between military means and humanitarian ends, that includes the enforcement of humanitarian norms among the just causes of war, and that returns us to a Grotian conception of legitimate authority to respond to atrocities. Right or wrong, these are the premises of a new just war theory.