Former Secretary of State Hillary Clinton admitted that she “could not have predicted” the effectiveness of the al-Qaida-affiliated terrorist group that seized control this week of two major Iraqi cities.

The Islamic State in Iraq and the Levant (ISIS), which has pledged allegiance to al-Qaida since 2004, captured the western Iraqi cities of Tikrit and Mosul, expanding its influence across the entire Sunni-dominated western region of Iraq in addition to Syria — where it remains one of the strongest rebel factions fighting dictator Bashar al-Assad. The group, led by enigmatic terrorist Abu Bakr al Baghdadi, evolved out of al-Qaida’s Bush-era presence in Iraq, adopting a version of its current name as early as 2006.

Clinton, the Obama administration’s first Secretary of State and a presumptive 2016 presidential candidate, admitted that she underestimated ISIS in an interview Thursday with NPR’s Terry Gross at the Council on Foreign Relations in New York City.

“So this is not just a Syrian problem anymore,” Clinton said. “I never thought it was just a Syrian problem. I thought it was a regional problem. I could not have predicted, however, the extent to which ISIS could be effective in seizing cities in Iraq and trying to erase boundaries to create an Islamic state. That’s why it’s a wicked problem.”