Alwaght - Juergen Todenhoefer's journey was a tough one: dangerous, but also eye-opening. The author traveled deep into ISIS territory -- the area they now call their "caliphate" -- visiting Raqqa and Deir Ezzor in Syria, as well as Mosul in Iraq .
Mosul, Iraq's second largest city, was taken by ISIS terrorist group in June .
Todenhoefer saw the realities of daily life under ISIS terrorist group, with all shops having to close for prayers in the middle of the day .
"There is an awful sense of normalcy in Mosul," he said in an interview with CNN .
" 130,000 Christians have been evicted from the city, the Shiite have fled, many people have been murdered and yet the city is functioning …".
He says there is an air of fear among residents: "Of course many of them are quite scared, because the punishment for breaking the Islamic State's strict rules is very severe ."
Todenhoefer told the enthusiasm the ISIS terrorists showed was one thing that 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… ."
Todenhoefer met one somewhat overweight recruit in a "safe house" who said he wears a suicide belt to every battle because he is too chubby to run away if he is cornered and would choose to blow himself up, rather than be captured .
ISIS terrorists also has a track record of abusing, torturing and executing prisoners of war. Todenhoefer was briefly able to speak to a Kurdish captive while in Mosul. The captive claimed he had not been tortured, but Todenhoefer said he found that hard to believe .
"This was a broken man," Todenhoefer said. "It was very sad to see a person in this state. He was just very weak and very afraid of his captors ."
Todenhoefer conducted the interview with the prisoner while several ISIS fighters stood guard. He asked the man whether he knew what would happen to him .
"I do not know," the captive told him. "My family does not even know I am still alive. I hope that maybe there will be some sort of prisoner exchange ."
Todenhoefer was also taken to see child soldiers outfitted with Islamic State gear and brandishing AK-47s. One of the boys seemed very young but claimed he had already gone to battle for ISIS .
"How old are you?" Todenhoefer asked .
"I am 13 years old," the boy replied -- though he looked even younger than that .
One of the most remarkable episodes of Todenhoefer's trip to the ISIS-controlled region came when he was able to conduct an interview with a German fighter who spoke on behalf of ISIS's leadership .
The man -- clearly unapologetic about the group's transgressions -- vowed there was more to come; he also issued a warning to Europe and America .
"So you also want to come to Europe?" Todenhoefer asked him .
"No, we will conquer Europe one day," the man said. "It is not a question of if we will conquer Europe, just a matter of when that will happen. But it is certain ... For us, there is no such thing as borders. There are only front lines" .
"Our expansion will be perpetual ... And the Europeans need to know that when we come, it will not be in a nice way. It will be with our weapons. And those who do not convert to Islam or pay the Islamic tax will be killed ."
Todenhoefer asked the fighter about their treatment of other religions, especially Shiite Muslims .
"What about the 150 million Shiite, what if they refuse to convert?" Todenhoefer asked .
" 150 million, 200 million or 500 million, it does not matter to us," the fighter answered. "We will kill them all ."
The interview became testy when they reached the topic of beheadings and enslavement, especially of female captives .
"So do you seriously think that beheadings and enslavement actually signal progress for humanity?" Todenhoefer asked .
"Slavery absolutely signals progress," the man said. "Only ignorant people believe that there is no slavery among the Christians and the Jews. Of course there are woman who are forced into prostitution under the worst circumstances .
"I would say that slavery is a great help to us and we will continue to have slavery and beheadings, it is part of our religion ... many slaves have converted to Islam and have then been freed ."
The ISIS spokesman blamed the beheading of captured Western journalists and aid workers on the policies of America .
"People should really think about the case of James Foley," he said. "He did not get killed because we started the battle. He got killed because of the ignorance of his government that did not give him any help ."
Even with recent gains by Kurdish forces against ISIS terrorists in Northern Iraq, Todenhoefer sees the extremist group as entrenched, building state institutions, and that it shows no sign of losing its grip in the main areas it controls in Iraq and Syria .
"I think the Islamic State is a lot more dangerous than Western leaders realize," he said. "They believe in what they are fighting for and are preparing the largest religious cleansing campaign the world has ever seen ."
