Personally, I think at the end of all this we will end up with a rump Syria, with Russia as guarantor, a Kurdish protectorate guaranteed by the US and an Islamist protectorate guaranteed by Turkey.Its a perverse twist, that Assad probably is the only player who is trying to preserve the integrity of Syria. As the myriad of rebel groups if they're had won, would never have re-united the nation together (see Libya for example). The outside players like Turkey or Israel, have ulterior motives which in a nation we frown upon would be seen as bad as saddam.
For once we can thank parliament for growing a backbone and stating clearly we want nothing to do with another failed war.
Sadly, we probably need to completely drain the swamp of combat power on all sides before the parties will accept the gains and losses of territory that will be necessary - as occurred in Bosnia.
Indeed, the more fighters the warring parties lose, the more quickly that settlement will come.