The jihadist ideologues of the Islamic State believe in a profound clash of civilizations. The extremist organization's propaganda recalls an old prophecy that the armies of Islam will fight the forces of “Rome” on the fields of Dabiq, a town in northern Syria. The victory there will prefigure the would-be caliphate's conquest of the West — or whatever contemporary stand-in for “Rome” you see fit — and lead to the end of history as we know it.

For that reason, the group has named its main magazine Dabiq. It calls its enemies — be they rival Syrian factions or Western powers — agents of Rome or infidel crusaders. And, for a time, it seemed that the Islamic State was even trying to goad the West into a confrontation near Dabiq. The militants chose to execute an American aid worker, Peter Kassig, in the town in 2014. “Here we are, burying the first American Crusader in Dabiq, eagerly waiting for the remainder of your armies to arrive,” Kassig's executioner said in a video that was released later.

Even after Turkish-backed militias drove the Islamic State out of Dabiq earlier this year, there's evidence that the militant group is still playing the long game and clinging to the prophecy, according to experts who track the group's online propaganda.

And perhaps the Islamic State — also known as ISIS — may have cause for excitement during a Donald Trump presidency. Both Trump and those in the seething online churn of the “alt-right” — a catchall term for a coterie of neo-fascists, white supremacists, ethno-populists, anti-feminists and other far-right extremists who cheered Trump's electoral victory on Nov. 8 — also embrace a clash of civilizations. The president-elect signaled as much in a major foreign policy speech in April, when he rejected the idea of “universal values” and trumpeted the promotion of Western civilization.

Stephen K. Bannon, the alt-right ideologue who is poised to play a key role in Trump's White House, has articulated a more coherent vision. In a 2014 event with Catholic conservatives hosted at the Vatican, Bannon declared that the West was in “the beginning stages of a global war against Islamic fascism” and framed the contest in religious terms — never mind that Muslims themselves are suffering the most from and laboring the hardest against extremist groups like the Islamic State.

Bannon placed the current fight in a long history of Christian vs. Muslim conflict and praised the toughness of earlier European kingdoms. (Of course, he did not acknowledge the equally long history of Christian and Muslim cooperation.)

“If you look back at the long history of the Judeo-Christian West struggle against Islam, I believe that our forefathers kept their stance, and I think they did the right thing,” he said and then invoked two famous medieval battles in which largely Christian forces in Europe repulsed Muslim armies. “I think they kept it out of the world, whether it was at Vienna, or Tours, or other places … It bequeathed to us the great institution that is the church of the West.”

Animated by these ideas, a whole realm of alt-right Trump supporters have imported the iconography of the Crusades and other medieval warfare into their memes and messaging. They quote the famous speech supposedly made by Pope Urban II in 1095, in which he called for the First Crusade to reclaim the “Holy Land” from Muslim rule.

“When an armed attack is made upon the enemy, this one cry be raised by all the soldiers of God: 'It is the will of God! It is the will of God!'" the medieval pontiff is believed to have said.

“Deus Vult” — or “God wills it” or “it is the will of God” — has become a kind of far-right code word, a hashtag proliferated around alt-right social media and graffiti scrawled in public institutions. Just consider this sampling of tweets before and after Election Day.

Whatever his own beliefs, Trump will arrive in the White House with the backing of myriad people crying out for a holy war.

Many far-right parties in Europe are celebrating the presidential victory of Donald J. Trump. (Jason Aldag,Ishaan Tharoor/The Washington Post)

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