Max Fisher of Vox has a persuasive list of what a mess Syria is in and how inadequate most proposed solutions are. He calls the peace talks doomed, a Sunni army a mirage, a no fly zone unhelpful. I agree. He also makes the telling point that America's allies Turkey and Saudi Arabia are making things worse.
I'd add another deeply troubling factor: the longer the war goes on, the greater the risk to Europe. The migrant crisis is already fraying the civility and unity of the European community and risks empowering groups who reject toleration and civil liberties.