MOSUL, Iraq -- The so-called Islamic state is crumbling.

Iraqi forces are now on the edge of Mosul’s historic old town - one of the extremists’ last strongholds



ISIS fighters are being killed and rounded up as prisoners.

But even as they lose on the battlefield, they’re still waging an online propaganda war, painting themselves as a band of holy warriors.

One ISIS defector, who met with CBS News correspondent Holly Williams in Turkey, showed just how powerful that propaganda can be.

An ISIS defector talked to CBS News’ Holly Williams. CBS News

He said he was a government worker in Jordan before he joined ISIS in late 2014, believing they had created a just Islamic state, but said he quickly realized he’d made a mistake.

“They kill anyone who argues with them,” he said.

He was surprised to discover their brutality yet joined the extremists even after he’d watched some of their vicious execution videos, including the beheadings of two American journalists.

“Why would you still join when you had seen them committing atrocities like that?” Williams asked.

“They said they were CIA agents, not journalists,” he said, “and they slipped a verse from the Quran into the video to justify the killings. No Muslim could object, or say this is wrong.”

An ISIS defector says that there are other members like him, and “if they’d known the truth, they wouldn’t have joined.” CBS News

He told Williams he paid a smuggler to get him out of ISIS territory last year. He’s far from alone in succumbing to the extremists’ propaganda.

“There are many people in ISIS like me,” he said. “Tens of thousands. If they’d known the truth they wouldn’t have joined.”

ISIS is losing the fight in Iraq and Syria, but that may not be enough to loosen its grip on the minds of its followers.