When I say impossible or nearly so, it is because it would require some random actor* to do the following:
- Convince Russia and Iran to stop supporting Assad. All we know about the international politics of intervention is that when outsiders pour in support on opposing sides, the conflict endures, which implies more casualties. So, we need to get the folks who are supporting Assad to switch sides. To be clear Assad is killing more Syrians than ISIS, so if you want to save lives ....
- Figure out which local ally is going to have heft domestically and have interests that line up with the outside supporters. Given what we know what happens when there are more veto players, um, not good.
- Find some kind of interest that will justify for the audiences at home (that is, within random actor x) the tax dollars, the loss of life, etc. that the intervention will cost.
- Staying power: any intervention in Syria would require not just a quick invasion, but an occupation/peacekeeping effort that would take at least twenty years (see Afghanistan).
* And don't kid yourselves: that random actor is the US. Nobody else has the capabilities to do the job. It could be the US+ as in NATO or as in US and coalition of the willing, but nothing meaningful can happen without the US.