Russia Supports Assad in Syria with an Army of Mercenaries
Russia is in Syria for the long run and they have a vested interest in seeing Assad triumph over the opposition. But, the issue of military casualties is sensitive in Russia because of the wars in Chechnya and Afghanistan that took the lives of so many young Russian men and women. Putin is using mercenaries in Syria because they don't have to report their deaths as they would for regular soldiers. And there are approximately 2000 to 3000 of them assisting Assad's Army right now. Russia also provides Assad with many of the supplies his army needs but which he can't purchase because of US and international sanctions. Russia's assistance to its old ally has been pivotal in Assad's resurgence and there is now little doubt that he will eventually retake most of the territory lost to rebel groups over the last few years.
America has long supported Assad's ouster, but Russia will remain in Syria to ensure his survival. President's Trumps recent statements about withdrawing US troops acknowledges the strategic situation; Russia [and Iran] have larger interests there and will define the future of Syria.