Well, I think our head of the intelligence community acknowledged that they underestimated what had been taking place in Syria. Essentially what happened with ISIL is that you had al-Qaeda in Iraq, which is a vicious group, but our Marines were able to quash it with the help of Sunni tribes. They went back underground. But over the past couple of years, during the chaos of the Syrian civil war where essentially you have huge swaths of the country that are ungoverned, they were able to reconstitute themselves and attract foreign fighters—who believed in their jihadist nonsense and traveled everywhere from Europe to the United States to Australia to other parts of the Muslim world—converge on Syria, so this became ground zero for jihadists around the world, and they have been savvy in terms of social media
At the end of the interview Obama's asked if he agrees with Clapper that the US overestimated the ability of the Iraqi military, the president answers in the affirmative.
Of course he places blame on the nation's intelligence community rather than the senior members of he administration who interpreted the data. Or Obama himself who on last week's 60 minutes, said that ISIS grew because Obama allowed the US to leave Iraq too early and didn't allow the US to get involved with helping the Syrian rebels--against the wishes of his national security force.