I think the use of chemical weapons is the game changer. If the US & allies turn the other cheek it shows those extremists that the use of these weapons empowers them. If Syrian government uses them on their own people wihout consequence then where's the deterent to use them again and again and against others?

An extremist with a cache of nerve gas or biologics is a different thing than an extremist with a dozen RPGs, a thousand machetes or a suicide vest or car trunk full of conventional explosives.

Not saying the US alone needs to be the world's policeman, saying that the US and the rest of the modern world (hopefully someday including Russia and China) are all in this together.