Diplomacy, diplomacy, diplomacy. Isolate all elements from external support. Work WITH Russia, Chine and the UN. Prevent arms supplies to ALL parties. Establish a ceasefire within Syria. Monitor with UN forces and air power. Strike any elements breaking ceasefire. Get all side to the negotiating table. Do NOT take sides (increases legitimacy of monitoring force in eyes of primary participants). Once internal elements are unable to resupply they will exhaust themselves. Include Assad government in any and all negotiations. Sit back and let them make their own peace/mess.

Alternative? Create a new Egypt or Libya and watch it all go to pot. Use air strikes and watch Uk and Us create new enemies for themselves, sit back and learn no lessons from history and then wonder how we got ourselves into this mess.