There is a limit to what can be done in internal conflicts such as the Syrian civil war. There is a chemical weapons convention that almost every nation has signed so there is an international norm against their use and agreement on their disarmament. This is not the case with conventional internal conflict. The Syrian regime will agree to disarm its chemical weapons to prevent bombing by NATO but removing conventional weapons or ending the conflict would be completely different; a much bigger operation which the Syrian regime could not agree too as it would mean signing their death warrant.