Some of the root to the Assyrian civil war, Assad's brutal use of force to hold onto power, and Russia's involvement. Assad is not a Muslim, he is an Alawites, and his father and then himself made the overture of courting Sunni influence to the Alawites in Syria so as to look like a legitimatized Muslim regime in the region. The map below is the result of the French Mandate for Syria, as you can see, the Alawites state is the same then as the Alawites region today. Being friends with Russia makes more sense not being a legitimate recognized sect of Islam.
Syria by Group or religious affiliation.
Recent map of who has control of what oil, and it looks a bit like a division of the country by religious and ethnic lines similar the old French Mandate. Turkey black market oil re-sellers have always bought boot legged oil from from Syria since before the civil war. Those re-sellers are still buying it from the Kurd's and ISIS, oil is oil. Once ISIS is gone who are Islamic, which all sides agree has to happen. The Kurd's will still have oil feilds to support their cause which Assad will want back. How do the resistance forces who are Sunni Islamic win against Assad and the Russians? Create an alliance with the Kurd's who are not all considered Islamic? Will the US switch to helping the resistance and Kurd's? This is super Bellyflop Krudtasticly stupid trying to apply a western view of conflict to peoples divided by religious and cultural sects in a country created by Europeans against their will for access to oil and possible mineral reserves almost a century ago.
So is this really a war between Assad, Sunni's, ISIS and the Kurd's over control of oil feilds? Without the oil exports and domestic energy production, not much else is in that region to support a population the size of Syria stuffed into cities next to one of the most arid deserts in the world that Palmyra and an oil field sits in.