A German authored article for Carnegie, with numerous links and ends with:
Considerations for cooperation with Assad overlook his weakness, to say nothing of the destruction it would bring to the anti-IS alliance. It could easily lead to the collapse of this coalition and would require policy choices that the U.S. in particular is not ready to take. For once, realpolitik appears to be the most solid deterrent against a deal with Assad.
Link:http://carnegieendowment.org/sada/20...ith-assad/hvru