Two state solution. We do Assad and the Palestinians get Syria for a homeland (Israel gets Israel). Behind closed doors we make clear that what the US giveth the US can take away so develop your new country, lay off the struggle - you lost
That's not going to work unless we demonstrate some real old testament outcomes for them. See Afghanistan, they've been fighting there over trivial scheist since before Alexander. The Brits had the same issues after WWI, they built a series of forts within mutual support distance of each other along the entire silk road and it was only constant warfare. If they do not understand that messing with us will bring a deity like area wide retribution that will kill their entire family tree they will not stop.
They are willing to blow themselves up over trivial gains or nearly pointless points. We need Pershing like solutions. We need to be able to say I'm happy to work with you and make your life better, or I'm happy to come and kill everyone you ever knew and bury you in a pit of pigs; your choice.