two powerful essays on this delicate topic; to put it in most basic terms: The US (and its allies Saudi Arabia, Turkey and Quatar) supported „moderate“ opposition groups in their quest to overthrow Assad and ultimately it turned out that many of those where not as moderate at as they had thought.

http://www.thenation.com/article/how-war-terror-created-worlds-most-powerful-terror-group/

http://www.theguardian.com/commentisfree/2015/jun/03/us-isis-syria-iraq