It’s really just Sunni vs Shiite, or more specifically Saudi Arabia vs Iran. The Saudis, clever as they are, long since sensed where the wind was blowing. They used their oil wealth to buy off the US. As a result, the US military went on foreign adventures to do their bidding. The Saudis also cleverly leveraged an US-led embargo against Iranian oil, depriving their rival of the asset with which they could have bought influence in the world. Unfortunately, the Saudis overplayed their hand, choosing to strike against Iraq in retribution for 1991 and as a way of making a buffer state against Iranian influence. However, the US is now too weak to invade Iran, much as the Saudis would want. Instead, Syria is playing out as the proxy battlefield for these two bitter oil-rich enemies. It’s the ultimate prize – a wealthy country steeped in history ruled by Shiites but with a Sunni majority. The Sunni coalition want to topple another domino. The Shiite militias want to prop up their kin, who has promised them “anything” in return for their help.
It’s a conflict that most sane countries would want to stay out of. With any luck, if the US were to intervene, both parties would band together to expel the “invaders”.