ISIL (or something like it; the name/identity couldn't be predicted) as a substantial force with territory, was a foreseeable consequences of the US invasion of Iraq, the inadequate allotment of troops for occupation compared to pre-invasion expert-identified requirements, and bungling of details of the occupation like de-Baathification, particularly the wholesale and disorderly dismissal of security services.
The US (and specific US policy makers) are directly responsible for that.
The US (and specific US policy makers) are directly responsible for that.