It's not as if Syria is defending their use of chemical weapons

‘They must be really bad if even Hitler wouldn’t use them’ by Ezra Klein

EK: So that makes it sound like there’s no reason to hit Syria. 

RP: I would word it this way: Bombing Syria would be the strongest possible upholding and reinforcement of the norm. Will the norm fall by the wayside if that doesn’t happen? No, I don’t think it will. But if there was a strike to enforce it, that would be a watershed moment in many respects. Norms regarding warfare have often been quite effective, like the treatment of prisoners of war. They’re sometimes violated, of course, but a lot of them are treated with a minimal level of decency. These norms trudge on, despite violations, because of beliefs about reciprocity and decency. One violation does not destroy a norm. What matters is how people respond to it. 

And you’ll notice something strange about this episode. It’s not as if Syria is defending their use of chemical weapons. They’re denying it. And that helps contribute to the notion this is an unacceptable process. In World War I, the Germans argued that gas might be more humane than bayonets or getting blown up. Some people think that the Bush administration’s view on enhanced interrogation techniques struck a real blow against norms against torture. No one is defending chemical warfare. All the dynamics here have served to highlight that this is a salient norm in global politics today.