The same process we saw on the Left in the last decade is now repeating itself on the Right. Ideas that begin on the extremes — all Muslims are aliens and potentially dangerous threats to Christian Europe, in this instance — move into the mainstream media. Theresa May does not take on the normalisation of prejudice today, just as Gordon Brown and Ed Miliband did not take on the far Left as it grew in influence before 2015. If and when Brexit turns sour, I can see how what’s left of  liberal conservatism will be swamped by the Right’s determination to blame foreigners rather than its own folly and we will have a Tory Corbyn, but this time with the power of the state behind him or her.

Tip-toeing around Islamic State in these circumstances is politically understandable but intellectually indefensible, as Tom Holland’s Channel 4 documentary Isis: The Origins of Violence made clear. I should declare an interest and say that I know and admire Holland, and my admiration grew as I watched his work. Scholarly and sympathetic, he explored the roots of Salafism without compromise. It may be tactless to argue that you can find Koranic authority for treating Jews and Christians as second-class citizens or for massacring and enslaving the “satanic” Yazidis. But the justifications are there in the Koran and versions of Sharia law, even if as, Holland says, the vast majority of Muslims would condemn Islamic State’s “monstrous crimes”.

To say that Islamic State isn’t Islamic in these circumstances  is like saying Opus Dei isn’t Catholic, or ultra-Orthodox Jews aren’t Jewish. It is a propaganda move because they all feast on obscurantist traditions in their religions. It may be a necessary propaganda move for liberal Muslims, Christians and Jews trying to reform their creeds. But for countries at war with Islamic State, not just in the Middle East but on the streets of our cities, it is wishful thinking we cannot afford. 

When you are engaged in a struggle, you have to be clear-eyed about the motives of your enemies. Yet the default mechanism remains to explain away religious totalitarianism by putting it in secular terms we can understand: economics, Western foreign policy, and poverty. Any explanation of apocalyptic religious violence will do as long as it refuses to recognise that totalitarian ideas have a life of their own.

Holland tried to remedy our ignorance by invoking the terror of the French Revolutionaries. He could equally have looked at Communism or fascism. I doubt many of his viewers would have seen the connection. One of the strangest aspects of modern Europe is that although histories of Communism and fascism are everywhere, hardly anyone understands that millenarian ideologies can exist in changed forms now. The apocalyptic belief in a world cleansed of impurity when the kufr are annihilated is not so different from a heaven on earth achieved when the bourgeoisie or the Jews have been slaughtered. You only have to look at the Western-born and educated Muslims who flocked to Syria to see the same messianic spirit, the same belief in the purifying power of violence.  We see it, but we don’t recognise it, let alone understand it. And without understanding we will not be able to fight it.

Holland ended on a gloomy note. Millions of Muslims believe in democracy and human rights, he concluded with words even the BBC might have broadcast. But Holland understands what the BBC does not. Rival traditions in Islam do not take other Muslims’ modernity as a sign of defeat and accept that they are now just “so-called” Muslims, but as a reason to scour their religion clean of such impurities. After the Caliphate in Syria falls, which it will, they will turn their attention to what IS calls the “grey zone” of Muslims living in the West. I am not sure we are ready for them.