IMO, Putin's presence in Syria is an attempt to put back a semblance of stability into the region. Not merely social or military stability, but also economic stability. Without petro dollars the entire region would settle into chaos.
I also think he's playing a shell game with Iran, until he cleans up Obama's mess in Syria. Why fight a two front 'enemy' if you don't have to?
We all know that the USA is behind the anti-Assad rebels in Syria.....as if we didn't already learn that in the Middle East in Iraq, Egypt and Libya... dictatorships WORK.
Furthermore, the Russian and Iranian rules of engagement are "KILL THEM ALL", instead of the USA's concept of dithering and self-defense on patrol.
It's my feeling that the 150,000 Russian military draft call is just in case NATO goes insane...and possibly also to construct a military presence in Syria like we do/did in Germany, Korea and Japan.
Let's all admit that Putin would never imitate Reagan, should 300 Russian soldiers be killed in their barracks. He would never retreat.