Context: Though primarily remembered for sacking Constantinople in 1453, because Ottoman sultan Mehmed II was only twenty-one years old then, he still had many good decades of jihading before him. He continued expanding into the Balkans, and, in his bid to feed his horses on the altar of Saint Peter's basilica – Muslim prophecies held that "we will conquer Constantinople before we conquer Rome" – he invaded Italy and captured Otranto in 1480. More than half of its twenty-two thousand inhabitants were massacred, five thousand led away in chains.

A little remembered event that occurred 538 years ago today – the ritual decapitation of 800 Christians who refused Islam – sheds much light on modern questions concerning the ongoing conflict between Islam and the West.

To demonstrate his magnanimity, Mehmed offered freedom and security to 800 chained Christian captives. All they had to do was embrace Islam. Instead, they unanimously chose to act on the words of one of their numbers: "My brothers, we have fought to save our city; now it is time to battle for our souls!"

Outraged that his invitation was spurned, on August 14, on a hilltop (subsequently named "Martyr's Hill"), Mehmed ordered the ritual decapitation of these 800 unfortunates. Their archbishop was slowly sawed in half to jeers and triumphant cries of "Allah akbar!" (The skulls and bones of some of these defiant Christians were preserved and can still be seen in the Cathedral of Otranto.)

Now consider what this event says about current realities.

First, whenever Islamic individuals or organizations engage in violence against non-Muslims – and cite Islam as the reason for their behavior – we are instantly told the exact opposite: that they are mere criminals and psychopaths, that their actions have "nothing to do with the reality of Islam," to quote John McCain.

Yet it was not just run-of-the-mill "Muslims" who committed atrocities atop Martyr's Hill, but the official leader of Sunni Islam – the sultan himself, who always had a pack of Muslim ulema – clerics, scholars, and muftis – to guide and confirm his decisions vis-à-vis infidels (including massacring those who reject Islam).

Nor was Otranto an aberration. As documented in my new book, Sword and Scimitar: Fourteen Centuries of War between Islam and the West, Islam's official leaders and spokesmen – from sultans and caliphs to ulema and sheikhs – have always spoken and acted just like the Islamic State (or rather vice versa).

Also interesting to reflect on is how even then, over half a millennium ago, Western nations preferred to engage in denial and wishful thinking over coming to grips with reality or aiding their beleaguered coreligionists. Soon after the Otranto massacre, Pope Sixtus IV chided an indifferent West accordingly:

Let them not think that they are protected against invasion, those who are at a distance from the theatre of war! They, too, will bow the neck beneath the yoke, and be mowed down by the sword, unless they come forward to meet the invader. The Turks have sworn the extinction of Christianity. A truce to sophistries! It is the moment not to talk, but to act and fight!

Such laments were not uncommon; nearly a century later, in 1565, as a massive Islamic armament was sailing over to besiege the tiny island of Malta, Pope Pius IV complained that the king of Spain "has withdrawn into the woods and France, England and Scotland [are] ruled by women and boys."

Finally, and not unlike today, whereas the mass of Western people were ignorant of Islam's doings, a minority were always keenly aware, including from a historical perspective. Consider Sebastian Brant (b. 1457)'s "Ship of Fools," a satirical poem on the gradual nature of Islam's advances against a "sleeping" Christendom:

Our faith was strong in the Orient

It ruled in all of Asia in Moorish lands and Africa

But now [and since the seventh century] for us these lands are gone...

We perish sleeping one and all

The wolf has come into the stall

And steals the Holy Church's sheep

The while the shepherd lies asleep

Four sisters of our Church you find

They're of the patriarchic kind

Constantinople, Alexandria, Jerusalem, Antioch

But they've been forfeited and sacked

And soon the head [Rome] will be attacked.

As the poem's continuity suggests, learned Europeans saw the Ottoman scourge as the latest in a continuum of Islamic terror, for whereas the Arabs were "the first troops of locusts" that appeared "about the year 630," to quote a contemporary English clergyman, "the Turks, a brood of vipers, [are] worse than their parent ... the Saracens, their mother."

The same observations of continuity can be made about the Islamic State and every other jihadi organization.

Editor's note: A portion of this article is excerpted from the author's new book, Sword and Scimitar: Fourteen Centuries of War between Islam and the West. All quotes are sourced there.