Stay out of civil war
I object to the United States becoming involved in the Syrian civil war even under United Nations auspices. I do not believe that there is any chance of forming a coherent, friendly new government out of the various insurgent parties. Arming any of these forces is analogous to shipping scrap iron to Japan prior to Dec. 7, 1941.
Although the use of chemical weapons is abhorrent to all civilized people, I have not heard of any planned course of action that would prevent further uses. In fact, dissolution of the existing regime would only spread the current stocks of the weapons among the various insurgent forces and their use God knows where or upon whom.
The entire Middle East is a powder keg with many internecine conflicts that we of the West do not understand. Also, several of our European partners have old colonial and/or business interests or grudges that could lead us down paths better not traveled. It will be very difficult to predict all of the possible outcomes from our intervention in the area. The results could easily be far worse for the local inhabitants and the world than the current condition.