ISIS-style extremists are not new: they are indeed a more obvious part of human history than Western-style liberals. They are, however, newly powerful.

Take the plight of Assyrian Christians driven from their homes in Mosul. Their grandparents had the same experience, but worse: 300,000 of them were massacred a century ago at the instigation of the Ottoman authorities (as described in Joseph Yacoub’s new book, The Year of the Sword).

That was not unprecedented. The Ottoman Empire just 20 years before had sent an army to kill or convert the Yazidis, just as ISIS later would try to do. It had also done the same in 1838. The sanction for this cruelty was a fatwa from the revered 14th-century cleric Ibn Taymiyyah, which licensed the killing of heterodox groups like the Yazidis and Druze.

Is this, then, a purely Muslim phenomenon? Why, no. If we look back far enough, we will see Christians behaving similarly. “Kill them all,” said papal legate Arnaud Amalric in 1209, faced with a town of 20,000 Catholics and heretic Cathars. “God will know his own.”

There is something in human nature, with apologies to Robert Frost, that does love a wall. We enjoy the feeling of being among our own, and if there is a visible enemy then those feelings of closeness become even stronger and dearer.

Such feelings can exist even in the most prosperous and stable societies. In ones where a victory for one community may mean death or poverty or humiliation for another, the feelings are greatly intensified. Then add to the mix weak and unenlightened governments and Islamist movements which, instead of repressing communal hostilities, exploit them to their own advantage. Add, too, a society where religious identity is much more public, and much more political: where a rejection of Islam is regarded as not only wrong, but disloyal to the state.

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