Sorry, but this is a civil war at best, and a regional conflict at worst. It will not affect US oil shipments. It will not affect US property. It will not affect US lives. (none of those in substantial ways, at least)
So, we stay the f**k out.
I understand the precedent thing. Allowing use of chemical weapons will encourage others to use them. I don't really care.
The only precedent that I really care about are the 3 things mentioned above. If you affect US trade (specifically, oil), US property or US lives, then we will NOT allow a precedent to be set by not reacting. We will react. Stay away from those things, and you can go all Khmer Rouge on your own people. It's sad. But, losing American servicemen lives is sadder.