See also: this US intelligence report drafted in 2012
I had to leave Section 3 in because it's a doozy.
Imagine a title like "The Renewing Facilitation of Terrorist Elements from all over the Arab World Entering into Iraqi Arena", having it entirely redacted, and having it come directly after what they said about the establishment of a Salafist principality lol.
Also for further information:
Who Is To Blame For The Rise Of ISIL? featuring former head of the US Defense Intelligence Agency
Then there's how the US funnels money into Saudi Arabia that just happens to end up going to Salafist insurgent groups.
Also I've heard of stories in interviews from people who were NATO axis cannon fodder deployed in West Asia where the US and its allies would receive an order from the top of the chain to abandon their caches and armoured fighting vehicles without any reason or justification for it, only for that position to be immediately overrun and looted by groups like ISIS. And also ones where they arm and train militias who are under a banner like the FSA (itself pretty dubious anyway) only to discover that these militias were either flying a false flag or they immediately defected to ISIS or similar afterwards. Idk the truth of all that and I doubt that we will ever get much clarity on the issue but it's pretty jarring.
Just for fun, here's a good short history of Salafism through the lens of history and religious studies. (I really need to empty the contents of my brain which are associated with Ibn-Taymiyya to someone whose big thing is history, politics and religion to get their take on what's been rattling around up there...)