Having a Naval base there, tends to do that.
The Russians have abandoned lots of naval bases before, including the excellent one built with US tax dollars during the Vietnam War at Cam Ranh Bay. It's just that Russian client states have been falling like dominoes ever since the Cold War ended, mainly because we backed the opposition while they stopped funding their client regimes. Their welfare cases have become our welfare cases.
For what are probably reasons of national pride, Russia appears to have decided that Syria is somewhat important to them. Nobody knows how deep that support is, but if they feel it to be important enough to go nuclear, there is no prospect of American intervention, mainly because they would tell the occupant of the White House and he will back down.
Could a nuclear exchange occur over a piddling issue like Syria? Stranger things have happened. Who could have predicted that the assassination of Austria's crown prince would result in tens of millions of European dead and the destruction of Europe's ability to maintain its overseas empires, both in manpower and economic terms? The one thing Marx said that I've found amusing is his view that "history repeats itself, first as tragedy, second as farce". During WWI, the Great Powers were at least fighting over worthwhile things - the possession of vast overseas empires. Fighting a nuclear war over the right of Sunni Arabs in Syria to exterminate its minorities seems to define the essence of the word "farce". But as history has shown again and again, man does not live by bread alone. Napoleon's Grand Armee won victory after victory against numerically superior opponents based in part on the principle, as Napoleon put it, that a soldier will fight long and hard for a bit of colored ribbon.