Assad has had his remaining security forces retreat to the major cities or rural areas (in the southwest along the coast) that are largely Alawite. There is some talk among Alawite diehard of retreating to this coastal strip and declaring an independent Alawite state, under the protection of Iran and, perhaps, Russia. It's unlikely that Turkey, the United States or any other states in the eastern Mediterranean would tolerate such an arrangement.
Why would that abandonment of the bulk of Syria be the trigger for intervention?
If Russia, Iran, and perhaps China stand with Assad, how is that a trigger for war?
If Assad brings in air defense and chemical weapons and delivery systems into the rump Alawite state, how is that less of a deterrent for outside intervention?
I mean, the West tolerates a lot of bad behavior in a lot of bad states. Why is this intolerable?
I'd say that an Assad retreat to the mountains shouldn't stop us from trying to end his rule, but if the Alawites want their own state, I see no reason to fight over it.