Could Assad be persuaded to step down if he and is family is given immunity from prosecution? Surely there's some ambitious crony in the Syrian government willing to take the top seat so the big powers won't have to ally with someone so ... tainted.
That's what I would try if I were US President. Just get someone who isn't known as part of the family of the butchers of Syria, and then go from there. Unfortunately, I think the Assads love power too much, and they've invested too much of their efforts, money, and influence to stop now. If the hypothetical ambitious crony could be propped up and convinced to purge the Syrian government of the Assads though, that might work.
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