Also the funding of the Syrian rebels by Obama and other Western nations. ISIS was just one of the groups in the Syrian civil war, a bunch brutal and crazy idiots, that would have never amounted to anything.
At the same time CIA wanted to destabilize the region, to kick the currently Russia-friendly government out (and as a consequence kick Russia out of its pretty much only naval base in the region). So they kept funneling funds, training and weapons to the "moderate" rebels.
Of course all those weapons (along with many left behind in Iraq), and all those "moderate" rebels switched to ISIS. Now you can either say CIA is stupid or malicious. Stupid because despite the $100B in funding, they couldn't foresee it and it was a surprise, or it was pretty much the plan.
One can say, at the end of the day what are they (and many other military and intelligence) branches will do if is there is no new threat to worry about. "No drones or SIGINT targets, no promotions and paychecks". Both for military industrial complex and the government employees alike.
Either way I find it interesting how this was spun as a surprise to the media: "We had no idea why ISIS grew so fast, and managed to occupy so much territory. Not really sure how they got those modern tank killer missiles they keep making videos with...".