Any action against Syria will also bring Iran and potentially Russia and China into the mix
In the not too distant future the Iran issue is going to come to the fore and we'll be faced with the same question again. When it does, the stakes will be considerably higher and we may not be able to stop one particular ally in the region acting unilaterally (especially if we do back down, as I suspect we now will), possibly with its own nuclear weapons. By putting a big marker down now in Iran's puppet state may make China blink and realize that their present stance is courting disaster. My only concern is that the Russian's are too stupid to blink; they've found our weakness and will exploit to it to the full.