A German author given rare access to territory run by Islamic State has told the BBC that the group is stronger, more brutal and harder to confront than he had expected.
Juergen Todenhoefer spent six days in the IS city of Mosul in Iraq, travelling there via Raqqa, in Syria.
Mr Todenhoefer said he found IS followers highly motivated and supportive of the group's brutality.
He said the spread of fighters meant they were hard targets for air strikes.
A former German politician, Juergen Todenhoefer is the only outsider to have travelled deep into IS territory and back. And, considering that several Westerners have recently been beheaded, he did so at terrifying risk.
In Mosul, captured with lightning speed by IS in June, Mr Todenhoefer saw how the group imposes its extreme version of Sunni Islam.
Posters instruct men on the right positions in which to pray and tell women how to fully cover themselves.
They must not, for example, wear clothes that "resemble those worn by infidel women or men".
Images on advertising hoardings have been blacked out, and a bookshop displays pamphlets and tomes on religious rulings, including how to treat slaves.
He met child fighters bearing arms for the "caliphate," and encountered recruits from around the world, including the UK, US, Sweden and Trinidad and Tobago.
Rule by fear
Mr Todenhoefer said he was struck by their brutal zeal, and the scale of their ambition to carry out "religious cleansing" and to expand their territory.
"There is an enthusiasm that I've never seen before in warzones," he said.
"They are so confident, so sure of themselves. At the beginning of this year, few people knew of IS. But now they have conquered an area the size of the UK. This is a one per cent movement with the power of a nuclear bomb or a tsunami."
Filmed by his son - with a permit guaranteeing their safety - his material gives the impression of a group busy entrenching their bureaucracy, relatively unperturbed by the threat of coalition air strikes.
"I had the impression that they want to show that the Islamic State is working," Mr Todenhoefer said.
On the surface, life - he said - looks more normal than he had expected. But all the city's Christians and Shia Muslims had already fled in terror, after IS militants took over.
The jihadists now have their own justice system - with IS flags hanging in courtrooms - and their own police enforcing strict Islamic law, although the local police chief told him that he no longer needed to administer violent punishments.
Fear, said Mr Todenhoefer, appeared to be an extremely powerful deterrent.
It was the conversations that he had with the militants who were escorting him, more than what he actually saw, that were most disturbing for Mr Todenhoefer.
He said he reminded the fighters that most chapters of the Koran began with the words "Allah... most merciful".
"I asked: Where is the mercy? I never got the real answer."
Mr Todenhoefer estimates that the city is now being held by a few thousand fighters. But, he says, they have made themselves difficult to target by spreading themselves throughout the city and no longer travelling in convoys to avoid coalition air strikes.
The author believes that IS is stronger in areas of Iraq which it controls, than in Syria. In Raqqa, for example - the headquarters of its so-called state - he says that Syrian President Bashar al-Assad is still paying the salaries of government employees.
Safely back in Munich, Mr Todenhoefer told us: "They are the most brutal and most dangerous enemy I have ever seen in my life."
"I don't see anyone who has a real chance to stop them," he said. "Only Arabs can stop IS. I came back very pessimistic."
Mr Todenhoefer was lucky to come back at all, even though he had negotiated access to the territory, via a German jihadist, for many months and carried permission issued from the "office of the Caliphate" - which protected him on several occasions.
"I was concerned at some points that they could change their mind," he said.
In the end, unsure whether they had indeed changed their minds and decided to take him and his son hostage, he had to run across the border into Turkey.
"I had to run 1,000 metres [half a mile] with our bags and all the things we had with us," he said.
"When we arrived, I had such an incredible feeling of happiness. I realised then that I had had tonnes on my shoulders. I called my family. And in this moment I realised it was not very easy what I had done."