German author and former politician, Juergen Todenhoefer spoke with CNN after travelling into territory occupied by ISIS including Syrian cities Raqqa and Iraq’s second largest city Mosul.

“I think the Islamic State is a lot more dangerous than Western leaders realize,” he said to CNN . Their main point of strength, he said, is the fighters’ willingness to die for Islam. “They believe in what they are fighting for and are preparing the largest religious cleansing campaign the world has ever seen.”

In an interview with Todenhoefer, a senior ISIS fighter warns the US and Europe. “We will conquer Europe someday. It’s not a question of us wanting but we will. We will kill 150 million, 200 million, 500 million – we don’t care about the number.”

Todenhoefer interviewed child soldiers, discussed beheading and slavery but told CNN the enthusiasm the ISIS militants showed especially stood out.

“When we stayed at their recruitment house, there were 50 new fighters who came every day,” Todenhoefer said. “And I just could not believe the glow in their eyes. They felt like they were coming to a promised land, like they were fighting for the right thing.”