US troops and allied Sunni militias defeated al Qaeda in Iraq during the post-2006 "surge" — but it didn't destroy them. The US commander in Iraq, General Ray Odierno, described the group in 2010 as down but "fundamentally the same." In 2011, the group rebooted. ISIS successfully freed a number of prisoners held by the Iraqi government and, slowly but surely, began rebuilding their strength.
ISIS and al-Qaeda divorced in February 2014. "Over the years, there have been many signs that the relationship between al Qaeda Central (AQC) and the group's strongest, most unruly franchise was strained," Barack Mendelsohn, a political scientist at Haverford College, writes. Their relationship "had always been more a matter of mutual interests than of shared ideology."