If chemical or biological weapons are used, I fully expect the USA to be there via NATO. Our President doesn't have the stones to do something like this on his own.
All that said, I say... let them fight it out themselves and see who wins. The human cost is not our problem unless it is our people. If we actually do fight there, what on earth would be gain and I feel we have to gain something. Iraq, Afghanistan, Libya, and Egypt have really worked out for us and our ideals whatever they are anymore.