Actually it is Iran and Russia against Saudi Arabia.
Iran and Syria are Shia, Saudi Arabia is Sunni. Syria is a Russian client state. Yemen and Syria are proxy wars between Iran and Saudi Arabia to see who will be the regional power.
So Isis is rabidly Sunni wanting to wipe out the apostate Shia Islam. Assad is Shia, although 70% of his country is sunni. Iran is Shia. The Saudis are Sunni. Isis has taken advantage of Syria’s civil war. Russia wants Assad to stay in power. Our President is partial to the Shiites. And this situation is even more complicated than I think I’ve stated so far.