I think Syria is part of the larger sunni/shia battle ground. Saudi Arabia and the other sunni states were alarmed by Iran's increasing influence in Iraq. So they created ISIS to act as a proxy force to block the shias. Assad is a Iran proxy, backed by hezhbollah militias, so Israel also wants him gone. Syria is Russia's one remaining middle eastern ally, and they have a naval base there, so they are all in for him.
The one obvious missing ingredient is the US. We have basically no interest there, which is why the calls to intervene make so little sense. Let the sunnis and shias fight it out among themselves, like they have been doing for hundreds of years.
Rubio says the world is safer when the US exerts leadership. How did that work out in Iraq, Libya, Egypt and a host of other places?