Another factor that must be born in mind when accessing the rapid growth of ISIS and its deep penetration into the heart of Europe, is the "West's" determination to overthrow the Assad government of Syria. It's obsession with Syria has created ISIS.
A declassified US document shows how the same United States government helped create a "salafist principality" (eventually ISIS or, more properly ISIL) in order to "isolate the Syrian regime", which was the objective of "the supporting powers". While admitting to all of this, the same document then contradicts itself by admitting it would be a "terrorist organization" that would create "grave dangers". Indeed it has. The mad dog is fed, now it turns and bites. Surprised?