The United States, by all indications, will soon become a belligerent in Syria’s civil war. The Syrian government's alleged use of chemical weapons to kill hundreds crossed a redline that U.S. President Barack Obama claimed a year ago would be the game changer, and the game for Washington, London, and Paris has clearly changed. Yet one thing has not: the international law governing when states may use force.

That is not to suggest that government lawyers won’t eventually try to offer some sort of legal benediction. News coverage suggests that administration officials are pushing them to do just that. And the lawyers will want to be helpful, particularly if the policy consensus for force is strong and the evidence for the regime’s responsibility for the attacks is beyond reproach.

But they should also be clear: It is the lawyers’ duty to provide their clients -- senior U.S. officials -- with legal, not moral, advice and counsel. The lawyers’ remit is not to say whether attacking Syria is the right thing to do, but to state what the law is, explain the positions adopted by the United States in similar circumstances in the past, and predict what the legal and institutional consequences of law-breaking might be.

So what is the law? The black-letter law on the use of force is quite simple: Under the United Nations Charter, the central treaty of the modern era and largely the handiwork of the United States and its World War II allies, states are generally prohibited from using force against other states unless they are acting in individual or collective self-defense or pursuant to an authorization of the UN Security Council. Over the post-war history of the charter, self-defense claims have proven most controversial. States -- especially the United States -- have sought to expand the situations that fall under the definition of self-defense.

But a case for self-defense in Syria would break the concept of self-defense beyond recognition. What concerns the administration, according to official statements, is the “moral obscenity” of a chemical attack on one’s own citizen. As awful as it is, there has been no attack (or the threat of attack) on the United States to justify individual self-defense or on allies to justify collective self-defense as a matter of law.

Given that a Security Council resolution seems unlikely, the United States is left without strong legal arguments for force. Some states, non-governmental organizations, and scholars have sought to craft exceptions to the requirement for Council authorization, usually under the rubric of humanitarian intervention or its contemporary form, the Responsibility to Protect (or R2P). Both exceptions spring from a moral position that states owe their citizens a duty of care, and when they violate it by committing grave crimes, force should be an available mechanism to halt or deter them. But neither exception has the force of law. The United States itself rejected humanitarian intervention as legal justification for the Kosovo war in 1999 even as the United Kingdom espoused (and still espouses) it, but the UK has few allies on the matter. R2P was blessed by the United Nations in 2005, but even there the United Nations decided that Security Council authorization was necessary for any intervention to qualify as legal.

Obama has also evoked norms against the use of weapons of mass destruction, such as the 1925 Geneva Protocol prohibiting use of poison weapons (to which Syria is a party). This prohibition may be strongly stated, but the treaty itself provides no basis for using force. Like many instruments of its time, it does not talk about the consequences of violation.

So, unless the Security Council authorizes action, the United States and its participating allies would be in violation of international law in using military force against Syria. Call it what you will: “illegal” if you are frank, “inconsistent with international law” if you are a lawyer, “difficult to defend” if you are a diplomat. They all amount to the same thing: No international law supports a U.S. attack on Syria, even in the face of mass killing by internationally prohibited weapons.

The United States will most likely seek some other means of justifying its actions. Its behavior in similar situations, when officials want to use force but have no obvious legal basis to do so, is instructive. Many commentators are pointing to the Kosovo war, for good reason, as the legal and political precedent in government lawyers’ deliberations. In 1999, with the war in Bosnia a very recent memory, the United States and its NATO allies perceived a major humanitarian disaster in the Balkans, with the alleged Serb ethnic cleansing of Kosovar Albanians. But the Russian Federation then, as now, refused to countenance any Security Council authorization of force, which forced NATO to consider an alternative international legal basis.

State Department lawyers, wary of establishing a legal precedent that other states could exploit in future conflicts, refused to give their legal imprimatur. Instead, they worked with policymakers to generate a set of factors that, in the specific context of Kosovo, provided justification (if not legal sanction) for using force. Those factors included the threat of a humanitarian disaster, disruption of regional security, and the paralysis of the Security Council. But they also relied on the former Yugoslavia’s failure to meet prior Security Council demands.

In the case of Syria, there are no prior Security Council demands. But it does seem that the United States may be heading toward a renewal of that general approach. Obama, in an interview with PBS, listed a set of factors with specific relevance in Syria, especially the perceived need to uphold the international norm against the use of chemical weapons.

From a policy perspective, the so-called factors approach that applied to Kosovo is attractive; it makes force seem legitimate even when not legal, and many policymakers care more about legitimacy than legality, particularly if there are no concrete legal consequences to action. But by suggesting that law and legitimacy are oppositional -- or more specifically, that the UN Charter’s framework is illegitimate to the extent that it allows some states to shelter and permit atrocious behavior by themselves or their allies -- this kind of legal sleight-of-hand damages the integrity of international law and its institutions, including the Security Council. As some powers grow in strength, such as China, the United States could regret having helped undermine the Security Council’s legal control over the use of force.

Finally, there is the question of consequences for this kind of law-breaking. Criminal liability is almost unthinkable. Though the International Criminal Court may have jurisdiction over illegal uses of force in the future, using force unlawfully now does not generate the same kind of criminal culpability under international law as provided for crimes against humanity, war crimes, and genocide. States do not generally (if ever) investigate and prosecute such uses of force by foreign leaders under universal jurisdiction statutes. Unlike with claims about Bush administration torture programs, few if any states would be able to address illegal uses of force in their national courts.

Obama administration officials could still vacation in Europe, in other words (though perhaps not Belarus). But policymakers should still be thinking about the legal consequences for the UN Charter system. Would the unlawful use of force against Syria make it more difficult for the United States to complain about others using force outside the doctrine of self-defense or Security Council authorization? Would it contribute to the development of a non-institutionalized norm of humanitarian intervention, under which any state could use force on its own terms? Or rather, would this kind of law-breaking help reinforce other norms of international law, such as the norm against use of chemical weapons or the targeting of civilians? Since lawyers for the U.S. State Department also work deeply with international institutions, they will want to consider whether the use of force in Syria could complicate other efforts and relationships across the United Nations.

In short, the United States is heading toward an intervention in Syria that administration officials clearly believe to be right, necessary, and humane. Their cause may be just. But it won’t be legal, and no creative amount of lawyering can make it so.