Bombing Syria for using chemical weapons against its own citizens would violate international law as it currently exists — let’s get that straight. But that doesn’t answer the question of whether the U.S. should do it anyway.

Some evils are so great that righting them requires violating laws that are inadequate to the situation, such as when the U.S. broke the same international law by bombing Serbia in 1999 to stop what looked a lot like genocide in Kosovo. The real question is: Should we break international law to send the symbolic message that use of chemical weapons violates, well, international law?

The legal analysis is surprisingly simple. If the United Nations Security Council authorizes force, international law allows it. Otherwise, unless acting in self-defense, a country or a group such as the North Atlantic Treaty Organization has no right to attack another.

The U.K. government issued a statement to the effect that international law recognizes a right to intervene proportionately when many lives are being lost — a version of what is sometimes called the international responsibility to protect, or R2P. The biggest trouble with this argument — the United Kingdom also used it when NATO bombed Kosovo — is that it doesn’t appear in any treaty or any definitive statement of international law.

The British lawyers might say that customary international law recognizes this right after the Kosovo bombings. But custom is supposed to become law when all or almost all states agree with it. And as we can plainly see from the Russian and Chinese opposition to bombing Syria, not everyone agrees. Even after Kosovo, the U.S. and U.K. went to the Security Council to seek authorization for bombing and then invading Iraq. The same was true of bombing Libya.

Illegality under international law shouldn’t end the discussion, however. Laws are made to be broken — especially international laws that create the possibility of horribly immoral results under some conditions. It would be monstrous to stand by and let hundreds of thousands or millions of people die preventable deaths because, say, Russia vetoes action in the Security Council.

The bombing of Kosovo was justifiable in moral terms — the saving of innocent lives — even if it was (cough, cough) illegal. If genocide in Rwanda or Bosnia or Cambodia or German-occupied Europe could have been prevented by unlawful intervention, it would have been the right thing to do. What is more, doing the right thing enough times might eventually change international custom so that the law does in fact change to allow or even require protecting the vulnerable.

But violent disobedience of the law shouldn’t be undertaken lightly. International law exists because its serves the interests of states and people. Almost all the time, it deserves to be followed. Breaking it weakens respect for the rule of law itself. It makes our treaties less meaningful and our commitments less firm. Breaking it makes us all a little less secure and safe.

It would be worth violating international law to save hundreds of thousands of lives in Syria — if we were confident we could actually do so. But that isn’t the Obama administration proposal. Instead, the U.S. and U.K. are talking about bombing in limited ways, with the goal of deterring further use of chemical weapons by Assad or other bad actors in the world.

It’s unclear whether the deterrent would work, of course. U.S. President Barack Obama’s warning about “red lines” obviously failed. But even assuming Assad and others might be deterred, is it worth the violation of international law to create this limited deterrence? Would enough lives be saved to justify the cost?

Numbers can’t fully answer this moral problem. Logic, however, can help. What’s wrong with weapons of mass destruction isn’t just that they kill lots of people. Assad has killed many thousands more of his citizens by conventional means. No, such weapons are particularly hateful because they violate international law. For more than a century we have realized that chemical or biological attacks are bad for everybody in war — which is why almost all nations on Earth signed treaties banning them.

So the principle behind bombing Syria would be this: Follow international law or face the consequences. How, exactly, can we send that message if the bombing itself violates the U.N. Charter? How can we credibly claim to deter egregious violation of international law by breaking international law ourselves?

The conclusion is painful, but I think also clear. If we can save many lives, we shouldn’t hold back from bombing Syria. But sending a symbolic message isn’t a good enough justification to deepen the precedent of violating international law when we feel like it. Especially when the symbolic message is about respecting that law in the first place.

Noah Feldman, a law professor at Harvard University and the author of “Cool War: The Future of Global Competition,” is a Bloomberg View columnist. Follow him on Twitter at @NoahRFeldman.

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