Martin Smith, the writer and producer behind the new PBS "Frontline" report "The Rise of ISIS," appeared on The Kelly File tonight to discuss the brutal terror group and how they grew so powerful.

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Smith explained that following the departure of U.S. troops from Iraq in 2011, the remnants of Al Qaeda in that country moved to Syria, finding space to incubate and eventually grow into ISIS.

"Without Syria, they really would not probably have become what they became," Smith said.

He added that high-level officials in the Obama administration have admitted that the White House botched the response to ISIS, hesitating about arming moderate Syrian rebels to keep the radicals from dominating.

"They knew that ISIS was a problem ... we didn't know how hollowed out the Iraqi security forces were. That's where they miscalculated," Smith explained, calling Iraqi forces a "ghost army."

Watch the clip above and be sure to catch "The Rise of ISIS" on PBS, Tuesday, Oct. 28, at 10p ET.

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