Obama and Syrian Chemical Weapons
The case against direct U.S. intervention never depended on believing that Assad was anything but a thug; rather, it rested first and foremost on the fear that intervention might make things worse rather than better. Specifically, it has rested on the interrelated concerns that 1) the fall of the Assad regime might unleash an anarchy of competing factions and warlords, 2) the opposition to Assad contained a number of extremist groups whose long-term agendas were worrisome, and 3) pouring more weapons into a society in the midst of a brutal civil war would create another Afghanistan, Iraq, or 1970s-era Lebanon. These prudential concerns still apply, irrespective of the weaponry Assad's forces have chosen to employ. And if his forces have used chemical weapons, then one might even argue that it raises the risks of intervention and thus strengthens the case against it.Beyond Walt's analysis, I would also say that it was never clear how most forms of intervention would work given the differences between Syria's protracted urban warfare and the fact Libya, the usual point of comparison, had something resembling an actual front in desert areas.
One case for intervening now is that Assad has broken a taboo against using chemical weapons that most of the world has tried to establish. He is certainly not the first; the chemical weapons attacks by Saddam Hussein are well known. All international norms, however, are situational. The international norm against genocide seldom leads to intervention.
Still, I think there should be a response, especially because President Obama went out of his way to declare prominently that this would be a red line provoking a response. If he did not have an idea of what such a response might look like, then he should not have pledged one knowing that the Assad regime was likely to go down this road rather than admit defeat.