The whole area is a tar baby.
There's nothing to be "won" there.
In a place like Syria, insofar as The West and its war with islam is concerned, it matters little who wins.
This sect of islam, that one, or another one -- the winners will be muslims.
Some will just be more... (choke)... "devout" than others.
They'll all remain under the control of the same "leader".
And he's nobody we can depose, because he died 1400 years ago.
But he's left his book of instructions -- his war plan -- for all of them to follow.
And the more closely they follow it, the greater the danger to The West.
Let the Syrians rein destruction down upon themselves.
Let them use whatever weapons they wish.
Poison gas -- should we really care?
(That's intended to be a serious question).
What's that saying, about how when your enemy is in the process of destroying himself, that's it best not to intervene?
Let them further reduce themselves from the surface population.
(I'm a grim pragmatist. Sorry if you don't like this post)
Another thought:
If we're going to line up against Assad, and want to get this over quickly, why not just send out a drone or two and ...?
Added question:
Once you take him out, who ya gonna get to replace him?