There's a bit of an economic cold war going on with Russia and NATO over military and economic spheres of influence. Russia supplies a lot of oil and gas to Europe so to undermine the Russian economy the Obama lead US government started fracking thus the oil and gas prices dropped and also gave them greater control over OPEC countries.
A proposed US backed pipeline from Qater was turned down by Asad who's rationale for rejecting the Qatar proposal was to be to protect the interests of Syria's Russian ally, which is Europe's top supplier of natural gas. 
When Assad turned it down the Obama and the CIA attempted to have him overthrown, then the US special forces started training and arming rebel militias who deceided to join ISIS rather than the US proxy Syrian free army.
Of course Russia has been supporting Assad to defend its economic interests and show it's allies that Russia's is loyal to its allies.
Of course there was also the 2016 coup attempt in Turkey probably a CIA to make sure they could be assured of a friendly more pliable goverment in Turkey giving the intrested parties in the Qatar-Turkey pipeline access to the pipeline infrastructure already in Turkey at a good rate.
Right now it looks like Russia and Assad will come out on top of the Syrian war.

Note: I fully recognise no one as unclean in the Syrian war not the NATO, US, Assad, the Russians, ISIS, Turkey, Iran, Saudi Arabia, Oman, Israel, Jordan etc except the innocent civilians.
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