BRUSSELS ― There is a saying, too often used in interpreting international relations, that my enemy’s enemy is my friend. Sometimes it proves true; often it does not.

Thirty years ago, the Afghan mujahedin were mistaken for friends of the West when they fought their country’s Soviet invaders. But how lazy that assumption seems now, given all that has since happened.

Syria’s deepening crisis, and the criminal use of chemical weapons there, has created a similar dynamic and dilemma. But the West need not risk making the same mistake and accepting the same false choices.

Begin with first principles. A chemical-weapons attack on the scale just seen in Syria must be deemed a game changer. Although possessing these weapons of mass destruction is technically not illegal, most states are parties to the 1993 Chemical Weapons Convention, which Syria has refused to sign.

So the answer to the question “What happens next?” cannot be “Nothing.” Principles of international law ― in particular, the emerging “responsibility to protect” doctrine and enforcement of the global ban on the use of chemical weapons ― dictate that some form of military intervention must occur in order to deter others from using WMDs, particularly against civilians.

But which measures are appropriate and genuinely useful? What is more likely to strengthen the West’s security, and what runs the risk of weakening it?

I believe that the fairest and simplest proportionate response would be to impose a no-fly zone on Syria. The proposal is particularly appropriate in the likely absence of any resolution under United Nations Chapter VII (“Action with Respect to Threats to the Peace”), owing to the (almost certain) cynical use by Russia and China of their veto power in the Security Council.

Of course, claims and counter-claims have been swirling in the aftermath of the appalling chemical-weapons attack on a rebel-controlled area east of Damascus. But, given the brutality of President Bashar al-Assad’s regime, no one can doubt the lengths to which it would go to hide its guilt. The five-day delay in allowing U.N. chemical-weapons experts to verify the attack gave Assad’s government ample time to conceal incriminating evidence, allow it to degrade, or destroy it with further shelling. The United States, France, and the United Kingdom are adamant that all the intelligence and eyewitness evidence points to the Assad government as the perpetrator of the attack.

There also is no doubt about the legitimacy of concerns about elements of the Syrian opposition. The al-Qaida-led and Salafist extremist groups in the rebel forces, such a Al Nusra, have proved to be just as vicious as the government and its allies, the Iranian proxy Hezbollah and Iran’s Revolutionary Guard. But the united view of Western intelligence services is that there is no evidence that these rebel groups launched the chemical-weapons attack.

In these circumstances, a no-fly zone would not only clear the skies of Syrian warplanes and missiles, thereby reducing the scale of the slaughter; it would also show Assad and his supporters that he truly is vulnerable. Generals ordered to use chemical weapons would have to reckon with the prospect that the regime could, actually, fall, and that they then might find themselves on trial for war crimes.

It would be better, of course, if Russia and China would allow the Security Council to do the job for which it was intended ― securing peace and preventing war crimes. By continuing to support Assad despite his use of chemical weapons, Russia’s standing in the Arab world has gone from patron to pariah. What little moral and political standing Russian President Vladimir Putin has retained in the rest of the world is also evaporating, as he will soon discover at the upcoming G20 Summit in Saint Petersburg.

But the world cannot hold its breath waiting for a change of heart by Putin and China, which is why a no-fly zone should be examined as a military option. In the aftermath of the Gulf War in 1991, a no-fly zone, initially proposed by British Prime Minister John Major, did not topple Saddam Hussein, but it did prevent him from carrying out further attacks from the air on Kurds in the north and Shia in the south.

Likewise, a no-fly zone in Syria would immediately restrict the Syrian government’s means of delivery of weapons of mass destruction. Some military experts may say that Syria’s air-defense systems are too sophisticated to suppress, making a no-fly zone too dangerous to enforce. But Israel has managed to attack Syrian territory twice ― destroying a North Korean-staffed nuclear reactor in 2007 and, more recently, striking a Hezbollah convoy ― with no casualties or loss of planes.

Mindful of this weakness, Russia has offered Syria its more modern S-300 missiles; but there is no evidence that they have arrived, let alone been deployed. And once Syria’s air-defense system is sufficiently degraded, it would be best if Arab countries ― Saudi Arabia, Qatar, and other Gulf states ― and Turkey used their air forces to police the zone. Any malicious wishful thinking on the part of Assad’s regime would be dispelled with every glance at the sky.

By Charles Tannock

Charles Tannock is foreign affairs coordinator for the European conservatives and reformists in the European Parliament. ― Ed.

(Project Syndicate)