To this point, most of the discussion about the legality of possible military action in Syria has focused on the concept of humanitarian intervention. The New York Times this weekend reported that some administration officials may be looking at the Kosovo conflict—when NATO attacked Serbian forces without UN authorization—as possible precedent for military action without United Nations approval, which presumably will not be forthcoming.
But both the general concept of humanitarian intervention and the specific example of Kosovo suggest a level of involvement for which Western governments have no appetite. When an outside force intervenes on humanitarian grounds, there is an implied committment to comprehensively address whatever is producing the humanitarian crisis. NATO did just that in Kosovo; it bombed Serbian military and government targets until Slobodan Milosevic eventually yielded control of the disputed province. At that point, the Western alliance dispatched a large and muscular force to police Kosovo and shepherd it slowly toward independence.
The analogue in Syria would be for Western governments to strike until the military balance turns and the Assad regime falls, and then to arrange an international force to help stablize whatever new regime emerges. Even with the horror of the apparent chemical attack fresh, that level of outside involvement appears highly unlikely. Even the more outspoken Western leaders, such as French foreign minister Laurent Fabius, have rejected the idea of dispatching ground forces.
Humanitarian intervention along the Kosovo lines remains a very distant possibility. There's another concept with deep roots in international law that might offer a more viable pathway for action: reprisal. In essence, the doctrine allows one party to respond to another's clear violations of international law by engaging in violations of its own. According to the International Committee of the Red Cross's elaboration of the doctrine, there are several strict conditions: a reprisal must be done for the purposes of inducing compliance with the law, it must be a last resort, it must be proportionate, and it must be approved at the highest levels of government.
The concept doesn't apply seamlessly to the Syria situation; any right of reprisal normally accrues to a party to the conflict, not to outsiders. And Western forces seeking to punish the Syrian regime presumably would not try to violate the rules of armed conflict themselves in their strikes (by, for example, targeting pro-Assad civilians). But in a broader sense, Western governments would be violating international law in order to defend it. More specifically, they would be skirting the rules on when you can use force in order to defend a key norm of how parties may fight: the ban on the use of chemical weapons.
Conceiving of a strike against the Assad regime as a reprisal rather than a humanitarian intervention has several attractions. It doesn't imply a committment to defeating his regime; it doesn't suggest a desire to side with his opponents; and it doesn't commit Western governments to end all abuses in the Syrian conflict or to nation-build in its wake. Instead, the message to the regime is simple, direct, and limited: if you use these hideous weapons, you will pay a price.