If you are at all interested in seeing the rationale among liberal types for intervention in Syria, watch this segment of Up With Chris featuring Jamie Rubin:
You see he gassed his own people with WMD and they might get into the hands of terrorists. We can't let the smoking gun come in the form of a mushroom cloud. We must disarm the tyrant because if we don't the terrorists will win. You know the drill. Very well. Too well.
But if you listen carefully, you'll see the more prosaic "liberal" reason: it's really fucked up in Syria and will be for a very long time so we should intervene because there's a chance we might make it fucked up for a shorter time. Or not. I'm going to guess that was basically the same rationale that liberal hawk types told themselves in the dead of night before we went into Iraq too. We're "good guys" just trying to help. But do we actually help? It's a matter of opinion, and certainly not one shared unanimously by the people we purport to be helping. If you look at Iraq today, I'd say that it's clear that if we sought to give the Iraqi people freedom we failed -- unless you think that living in a war zone for over a decade is freedom. (I don't think you can "give" people freedom, in any case...)
The world is an ugly place full of injustice. If it were up to me, and I could pick countries to liberate, I can think of a half dozen off the top of my head, starting with Burma. But sometime in the last half century I learned that war is very rarely a good way to "help" people and that the motivations for waging it are always far more complicated than that. Moreover, I just don't think the US is a very good "humanitarian" empire. Even if you assume we have the best of intentions, the US just doesn't have the right political structure to be a benevolent hegemon. I'm not sure any nation state does.
I don't know what will happen with Syria. I'm suspicious, as always, whenever someone starts talking intervention, especially when they use excuses that have been proven specious in the very recent past. Yes, it's always possible that they're right this time. But it's going to take more than just asserting that there's a threat of terrorism or that the region will be destabilized to convince me that we will do more good than harm. It's a horrible, bloody mess in Syria and it's terrible to feel impotent in the face of horrible bloody messes. But considering the huge risk we take of making things even worse, there is no doubt that we must be extremely skeptical of calls for intervention. The US has a bad hubris habit and considering our hot war success rate of the past 50 years or so, we really should take a good look in the mirror before we act.