The West won't intervene simply to avoid being blamed for a bad result rather than because we think good results are more likely if we stay out:
It’s the lack of rebel unity more than anything else that scares away the West which seems to believe that it is more prudent to let the rebellion run its course and then deal with the winner. If the fighting results in the country being partitioned and some areas becoming terrorist sanctuaries (that host groups attacking the West) then that will be dealt with. Meanwhile getting the Assad chemical weapons stockpiles neutralized is seen as more important.
I keep saying that it is better to try to defeat Assad and then deal with the threat of jihadis by helping the more acceptable rebels defeat the jihadis. The fact that influencing the outcome of this fight doesn't solve the problem once and for all shouldn't deter us.
Failing to intervene (and again, I don't mean by ground troops or even air power--just arm, train, and advise the more acceptable rebels to help them grow and win) will also require us to deal with the aftermath of the the war.
What are the possible outcomes, broadly speaking?
1) Assad could win.
2) The more acceptable rebels could win.
3) The jihadi rebels could win.
4) Syria could fragment with Assad (or maybe a post-Assad Alawite force) and various rebel factions holding their own ground.
5) Assad could be overthrown with rebels winning but holding their own ground in a fragmented Syria.
Face it, only outcome 2 is in our interests. So if anything else happens, we'll want to change the result or suffer from it.
And failure to intervene makes it less likely that outcome 2 is achieved, no?
Consider, too, that mass starvation among the Sunni majority in a longer war is likely a means for Assad to achieve outcome 1.
I think preventing outcome 1 is our highest priority. Assad has been our enemy and the blood of a lot of American troops is on his hands because Assad hosted Iraqi Baathists and jihadis who fought us in Iraq. Crushing an enemy would be a good lesson for all thug rulers to learn. Then we can work on getting outcome 2.
And remember, Turkey, Jordan, and Israel are right there, and will want to cope with whatever outcome there is. So there is a sort of safety net that will require them to act if they don't want to be acted upon by the Syria outcome.
Even the addition of chemical weapons doesn't change my priority since a victorious Assad will no doubt attempt to get revenge against the west for helping the rebels (remember the classic warning: if you strike a king, kill him). And that could include slipping chemical weapons to terrorists while holding out the Russian-defended excuse that some chemicals were stolen during the rebellion.
Don't pretend that refusing to act has no effect on the outcome.
