When you say 'atrocities are being committed in Syria, Something Must Be Done', then of course you look like a heartless isolationist to say no. But if you ask 'atrocities are being committed in Syria, would dropping bombs on the place make things better or worse?' then it's not so clear-cut.
I like the idea of going in there and bombing the murderous bastard to hell and back. But I like the idea of lots of things that would not necessarily work out so well in reality.
And I get the argument that letting people get away with using chemical weapons is not a good thing. But again, doing something for the sake of doing something isn't a good thing either, when you don't know what the consequences might be.