It’s all about a pipeline the Gulf States want to build to Europe, which the Russians do not want for obvious reasons.
Assad stands in the way of the pipeline.
Exactly. The trouble all started with the discovery of vast deposits of natural gas off the shore of Qatar. The Qataris and Saudis wanted to build a pipeline across Syria to Turkey and the Mediterranean, to expedite transmission to Europe.
Naturally, the Europeans supported this idea, because it would lessen their dependence on Russia and increase competition between suppliers. And also, naturally, the Russians were opposed because of the dependence of their economy on natural gas sales. So Assad, good Russian ally that he is, turned down the proposal for the pipeline across Syria.
This meant, as far as the Saudis, Qataris, and Europeans were concerned, Assad had to go. Their plan was to install some flunkey who could be easily bribed to approve the pipeline. But they wound up with ISIS, and now their problem is how to control ISIS, while at the same time getting rid of Assad.
The US, for its part, would like to get rid of Assad because they want to diminish Russian influence over the area. Also, they like the idea of the pipeline because it would lessen European dependence on Russian gas.