Obama was actively supporting any group that would topple a non-theocratic regime and when the they succeeded The power vacuum created havoc e.g. The Arab Spring. There was no good reason to try and topple Syria as they were a client state of Russia and Russia had no choice but to protect Assad. So since the US CIA was arming the good rebels (actually ISIS) that meant the only Russia was fighting ISIS as the US was only doing token military strikes against ISIS.