Trump really could crush ISIS. But what happens next could be worse.

What of the foreign fighters who came to join Isis in their thousands? Holmboe’s sources bring him news from the Danish citizens fighting for Isis, a mixture of ethnic Danes and immigrants. They all felt stuck in the shrinking caliphate because the way back to Europe was closed at the Turkish border. Nine of ten who did make it back to Denmark were no threat, he said. But some — the one in ten — were being instructed to smuggle themselves back ‘for operations’. ‘There could obviously be an upsurge in terror attacks in Europe and in the western world as revenge for the fall of the caliphate.’ Hundreds of British Muslims joined Isis too. When the caliphate is finally smashed to pieces, some of them will be coming home.

Though the physical caliphate might fall, Holmboe, said, the ‘virtual caliphate’ would remain. A document is circulating among Isis members through text messages and Twitter. Titled ‘The caliphate will not perish,’ it is a series of morale-boosting declarations by Isis leaders, living and dead. It was found by the researcher Aymenn al-Tamimi, who also obtained the sermon by the dissident cleric in Raqqa. ‘They remain defiant,’ he said of Isis. ‘Their message is that Islamic State will always exist as long as there’s the will to fight.’

In ‘The caliphate will not perish’ a member of the Isis Shura Council boasts that Muslims born in the US will become an army of conquest. ‘They are getting ready for the battle in their homeland. Today the caliphate is in Iraq and Syria. Tomorrow, it will be in the White House.’ The clash of civilisations is as important an idea to the Islamic State as to the Trump administration. Such boasting is absurd, but the fall of the Isis by the hand of America will be the ‘just cause’ for a new generation of jihadis. The end of the physical territory of the caliphate will not be the end of the idea of the ‘Islamic State’. That might be just as dangerous.