Bremer wrote a piece several years back that there were people above him who told him to dismantle the Iraqi army and fire all the Baathists. He agreed with it, though. There were issues with Shiites not being willing to follow orders of Sunni/Baathist superiors.
As for Al-Qaeda in Iraq, if you look up Ansar al-Islam was formed from a merger of two previously existing groups in the Kurdistan area of Iraq in September 2001 -- the same month as the 9/11 attacks. Ansar al-Islam became allied with Al-Qaeda then became Al-Qaeda in Iraq, which later became ISIS. So the roots of ISIS already existed before Bremer and before Bush started the Iraq War. ISIS didn't become a threat in Iraq until after the Obama-ordered pullout of Iraq in 2011. Remember, at that time Obama and Biden both said that Iraq was stable.
