I believe that if you want to end the Syrian civil war and tilt Syria onto a democratic path, you need an international force to occupy the entire country, secure the borders, disarm all the militias and midwife a transition to democracy. It would be staggeringly costly and take a long time, with the outcome still not guaranteed. However, without a homegrown Syrian leader who can be a healer, not a divider, for all its communities, my view is that anything short of an external force that rebuilds Syria from the bottom up will fail. Since there are no countries volunteering for that role (and I am certainly not nominating the US), my guess is that the fighting in Syria will continue until the parties get exhausted.
Meanwhile, wherever we can identify truly “good” rebels, we should strengthen them, but we should also be redoubling our diplomatic efforts to foster a more credible opposition leadership of reconciliation-minded Syrians who can reassure all of Syria’s communities that they will have an equitable place at a new Cabinet table — never underestimate how many Syrians are clinging to the tyrannical Syrian President Bashar al-Assad out of fear that after him comes only Hobbes or Khomeini.
That way, when the combatants get exhausted and realize that there can be no victor and no vanquished — a realization that took Lebanon 14 years of civil to come to — a fair power-sharing plan will be in place. Even then, Syrians will almost certainly need outside help during the transition, but we can cross that bridge when we come to it.
Here is the one alternative that will not happen: one side decisively defeating the other and ushering in peace that way — that is a fantasy.