Putin seems to have decided that Ukraine had become abjectly hostile to Russia and couldn't tolerate that. Lost his cool, made an impulsive dick move, and made it a self-fulfilling prophecy. Whether they were abjectly hostile before or not, they damn sure are now.
Syria wasn't about those puny shit hole ports IMO. It was about fighting Islamic fundamentalism. We have the opinion that, by golly, all you need is democracy and everything is fixed. The Russians, on the other hand, believe that the only unifying factor in the sandbox is Islam and everywhere there is democracy it's going to be taken over by the highly motivated religious nuts, so they supported Baathists and such, like we once did. Our project in Iraq came within an inch of becoming ISIL, so they may be right about that.
They are also much more connected with the ancient Christian communities in the region. Every place there has been democracy it's been hell for them when the fundies take over. Seems pretty clear the civil war against Assad was dominated by Muslim Brotherhood-links-to-AQers. Unlikely the fall of Assad would've gone well for those communities.
Edit to add: On topic, Putin will go to thermobarics before he goes to nukes. Until that happens we shouldn't get our panties in a bunch about nukes.
I don't see putty's worry about Islamy fundies. He a couple of degrees of Kevin Bacon away from that, now that he's not trying to keep the central asian Stan's in check anymore.
As for Nukes and no panties in Bunches - I've been saying that since Feb when this all kicked off. You might to tell Meli aka @ShortForBob . She's mo debbly got panties in supreme bunches over that.