What is good to realize is that Syria is considerably more important to Russia than to the US, so they will bid higher, likely all the way to global thermonuclear war.
A Russian retreat on Syria means increase of US pressure on Russia itself, all the way to break up of Russia. Thus, Putin retreating is not a likely option. A strike outside of the region that would lead to too fast an escalation is not likely either. Thus, in case of a US missile attack on Syria, retaliation will likely be done against US Navy And/Or bases in Syria (At Tanf) And/Or bases in nearby countries, like US bases in Romania or UK bases on Cyprus.
However, we may not have a GTW this time around, latest from Donald: White House comments on Syria - number of options besides missile strike
Russia will break up in a generation if not before Syria or not.
Demographics are killing them. And China would sure like Siberia and those resources.
And somehow Russia didn't attack all those countries when their clients Libya and Iraq went away.