For Vladimir Putin, Russia's military build-up in Syria is a potential diplomatic trump card and a handy way of diverting attention from Ukraine's frozen conflict. But it was a panicky realization that the Syrian government was being turned over on the battlefield that tipped the Kremlin's hand.

When it saw several months ago that Syrian government forces were retreating on several fronts at a rate that threatened President Bashar Assad, its closest Middle East ally, the Kremlin quietly decided to despatch more men, weaponry and armor, said diplomats and analysts.

"The situation at the front was pretty serious if not critical," said Ivan Konovalov, director of the Center for Strategic Trend Studies in Moscow. "Military assistance was needed on a larger scale; Russia responded."