Since neither a U.N. authorized military mission nor a unilateral American strike seem likely, what options are left?
One appealing option could be something along the lines of the Kosovo model. The Kosovo War in 1999 was entirely an air war in which no American soldiers were killed. The goal of the air campaign was to push Serbian forces out of Kosovo. Russia was allied with the Serbs so, as in the Syrian case today, there was no chance a U.N. resolution authorizing force would pass.
Instead, the war was conducted under the NATO collective security umbrella. Kosovo is, of course, in Europe, and NATO is a Europe-focused security alliance while Syria is the Middle East, so NATO action there would be much more problematic.
(A NATO force does fight in Afghanistan today, but that is only because one of its member states, the United States, was attacked on 9/11 from Afghanistan by al Qaeda, which triggered NATO's Article 5, the right to collective self-defense of the members of the alliance.)
If an air war were to be launched against Syria, one scenario could be that Turkey, a member of NATO, could invoke Article 5 because Syria has fired into its territory on a regular basis.
So far, Turkey has proved reluctant to invoke Article 5 but the reported large-scale use of chemical weapons by the Assad regime might change the calculus of the Turks.
A further source of legitimacy for military action could be the strong statement Tuesday by the Arab League that the Assad regime is responsible for the "heinous crime" of using chemical weapons. The Arab League is generally a toothless talking shop, which seemed to have surprised even itself two years back when it endorsed military action against Gadhafi. However, the Arab League has not endorsed any military action against Assad, although some of its members have privately urged the U.S. to take action against his regime in the past.
It is hard to believe that some kind of military action against Syria won't now take place, likely in the form of U.S. cruise missile attacks from ships in the Mediterranean.
Such attacks have the merit that they won't put U.S. aircraft at risk, which could well encounter problems with Syria's reputedly formidable air defense systems. And the operation will likely have participation by prominent NATO countries such as Britain and France and a tacit green light from leading members of the Arab League such as Saudi Arabia, giving it at least some semblance of international legitimacy.
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