Pope Francis is walking a knife edge—or perhaps, better said, the blade of a crusaders’ sword—as he tries to mobilize support for Christians and other minorities victimized by the ferocious partisans of the so-called Islamic state.

“Where there is an unjust aggression I can only say that it is legitimate to stop the unjust aggressor,” he told reporters throwing questions at him on the plane as he returned from South Korea to the Vatican on Monday.

“I underscore the verb ‘to stop,’” he told them. “I am not saying ‘bomb’ or ‘make war,’ but ‘stop him.’ The means by which he can be stopped must be evaluated. Stopping the aggressor is legitimate.”

The “him,” the “aggessor” in this case is the self-anointed “caliph” whose forces, formerly called ISIS, now dominate swathes of Syria and Iraq, and this is just the kind of reaction he’s been hoping for.

The crucifixions, the beheadings and the mass executions of men, the kidnapping of women to be sold as wife-slaves to so-called holy warriors, the destruction of ancient civilizations and cultures, from Assyrian statues to Yazidi villages, and the systematic intimidation, extortion, and murder of Christians—all have a purpose that can no longer be ignored:

Caliph Ibrahim, as he calls himself, wants to provoke a 21st century crusade against his Islamic State. He wants to force his enemies into a religious war arousing atavistic instincts rooted in the Middle Ages—the great glory days of Islam—that linger in the hearts of many Muslims around the world. And by every indication he is succeeding.

With each American bomb that falls and each drone that flies over the territories the caliph has conquered, he comes a little closer to that goal. Perhaps there really is no choice. As Hilaire Belloc, a poet, satirist, Catholic historian, and author of a book on the Crusades once wrote in his couplet “The Pacifist”: “Pale Ebenezer thought it wrong to fight, / But Roaring Bill (who killed him) thought it right.”

Certainly in recent days desperate men of the cloth, including some of the highest officials in the Catholic and Anglican churches, have played into the caliph’s hands by speaking out in support of the U.S. military action or calling for still more to be done—thus imbuing close air support with heavenly purpose that it probably can and should do without.

Even before Monday, Pope Francis used language interpreted by many as an endorsement of war.

There is a painful irony here. As recently as last month Francis was moved to tears by the carnage in Gaza, Iraq, and Ukraine.

“Never war, never war,” he said. “I am thinking, above all, of children who are deprived of the hope of a worthwhile life, a future. Dead children, wounded children, mutilated children, orphaned children, children whose toys are things left over from war, children who don’t know how to smile. Please stop,” said Francis. “I ask you with all my heart, it’s time to stop. Stop, please!”

In an August 9 letter to United Nations Secretary General Ban Ki-moon reported last week, Francis made an “urgent appeal to the international community to take action to end the humanitarian tragedy now underway.” But then as now the Pope did not quite call for military action even as he lamented news from Iraq that “leaves us incredulous and appalled.”

Then Archbishop Silvano Tomasi, the Vatican’s observer at the United Nations in Geneva, called not only for humanitarian assistance for those persecuted by the Islamic State, but explicitly for “effective military protection,” as well. Archbishop Giorgio Lingua, the pope’s ambassador to Baghdad, told Vatican radio that U.S. military intervention in Iraq in recent weeks “had to be done,” otherwise the caliph’s forces could not be stopped.

One senior Vatican official whose association with the pope goes back decades, told me privately these archbishops may have gotten out in front of the pontiff with their endorsements of military action: “There does not seem to be an alignment between the secretariat of state”—in charge of Vatican diplomacy—“and the pope.”

In Britain, meanwhile, Prime Minister David Cameron just started beating the drums of war in the pages of The Sunday Telegraph: “If we do not act to stem the onslaught of this exceptionally dangerous terrorist movement, it will only grow stronger until it can target us on the streets of Britain.” And this against the backdrop of calls to action by the Bishop of Leeds, who has raised “the serious concern that we do not seem to have a coherent or comprehensive approach to Islamist extremism as it is developing across the globe,” and the Bishop of Manchester, who called on the government to meet its “moral obligation” to Iraq’s Christians.

All this must be gratifying to the caliph, who has managed to transform himself and his followers from a group of fanatics roaming the Syrian and Iraqi desert only a couple of years ago to a force that seems to be challenging the whole of Christendom.

When Abu Bakr al-Baghdadi of ISIS proclaimed his Islamic State and re-branded himself as Caliph Ibrahim at the end of June, his calls for holy war were not veiled in the least, and his imagery was straight from the Middle Ages: “O mujahidin in the path of Allah, be monks during the night and be knights during the day,” he told his followers. “Take up arms, take up arms, O soldiers of the Islamic State! And fight, fight!” he exhorted them.

“Soon, by Allah’s permission, a day will come when the Muslim will walk everywhere as a master, having honor, being revered, with his head raised high and his dignity preserved,” said the caliph. “Anyone who dares to offend him will be disciplined, and any hand that reaches out to harm him will be cut off. So let the world know that we are living today in a new era…The world today has been divided into two camps and two trenches, with no third camp present: The camp of Islam and faith, and the camp of kufr (disbelief) and hypocrisy—the camp of the Muslims and the mujahidin everywhere, and the camp of the Jews, the crusaders, their allies, and with them the rest of the nations and religions of kufr, all being led by America and Russia, and being mobilized by the Jews.”

“This is my advice to you,” said the self-proclaimed caliph. “If you hold to it, you will conquer Rome and own the world, if Allah wills.”

Apparently, Rome and the world are taking him seriously.