VICE covered this in a really fascinating Special Report (Ep 18, Fighting ISIS - if you can, sign up for the 1 month free trail to HBO Now, it's worth the watch - or download it or watch clips on youtube). They actually sat down with Ambassador Ryan Crocker (who worked with Gen Petraeus during the war, and was tasked with "turning things around" by GWB) who admitted it was a colossal fuck up, even going so far as to say that more people die per day in Iraq now than they did under Saddam. Further, he states that we never faced a terrorist threat from Iraq before the invasion, while now there is a legitimate risk of terrorism.
Vice Debrief for the special report: https://www.youtube.com/watch?v=Kae-nng77yE
They basically fired ~500,000 who served in the Iraqi Army and police force - at which point they were angry, unemployed, and essentially ripe for joining the enemy faction (AQ/IS). Much of ISIS's leadership is said to be comprised of some of the leaders from the Iraqi Army. The commentary provided by the captured ISIS Jihadist members they interviewed is quite striking as well.
Further, the various groups that had been fighting ISIS (including the Anbar tribal fighters) that wound up declaring allegiance to ISIS when they lacked the support (either from Baghdad, or Washington) to continue fighting back.
Finally, the US didn't recognize the election results in 2010, and continued backing Nouri Al-Maliki (who they'd chosen to lead as PM in the first place) - which had caused its own problems through sectarian policies. Meaning, the Shiites retained power, after they had already turned the tables and persecuted the Sunnis (who had, IIRC, held power/favor under Saddam. Further, the group that ISIS and Al-Q are primarily comprised of). The disaffected Sunni populations turned out to be a great recruiting ground for ISIS.
A clip from a Charlie Rose interview with Ben Anderson, regarding his experiences reporting on Iraq/Afghanistan: https://www.youtube.com/watch?v=oPYOUIABYSk
Note: This is just my layman's take on the complexities of Iraq. Feel free to expand or correct me in areas if I'm wrong, I'm definitely not speaking with any sort of authority on the matter.