Can we really understand anything about Islamic State by looking at the centuries-old struggle between Islam and Christianity, asks Professor of Religious Studies Carole Cusack.
In 1996, late US political scientist Samuel P. Huntington published the book The Clash of Civilizations and the Remaking of World Order. Following the collapse of communism in 1989, he argued, conflicts would increasingly involve religion.
Islam, which Huntington claimed had been the opponent of Christianity since the seventh century, would increasingly feature in geopolitical conflict.
So, it wasn't particularly shocking when, after the September 11 attacks on the World Trade Centre and the Pentagon, the then-US president, George W. Bush, used the term "crusade" to describe the American military response.
Framing the subsequent "war on terror" as a crusade acted as a red flag to journalists and political commentators, who could treat the events as simply the most recent stoush in a centuries-old conflict.
The actual Crusades (1096-1487) themselves evoke a romantic image of medieval knights, chivalry, romance and religious high-mindedness. But representing them as wars between Christians and Muslims is a gross oversimplification and a misreading of history.
That there were wars between Muslims and Christians is certainly true. After the death of Abu Bakr (573-634), the Prophet Muhammad's father-in-law and first caliph, the second Caliph Umar (583-644) sent the Islamic armies in three divisions to conquer and spread the religion of Islam.
Whole regions that were Christian fell to Islam. The Holy Land, which comprised modern-day Palestinian territories, Israel, Lebanon, Syria and Jordan, for instance, was defeated. And Egypt was conquered without even a battle in 640.
The ancient and vast Persian Empire, officially Zoroastrian in religion, had been conquered by 642. Weakened by war with the Christian Byzantine Empire, Persia was no match for the Muslim forces.
Muslim armies marched across north Africa and crossed the Straits of Gibraltar into modern Spain, eventually securing a large territory in the Iberian Peninsula, which was known as Al-Andalus (also known as Muslim Spain or Islamic Iberia).
They also marched across the Pyrenees and into France in 732, the centenary of Muhammad's death. But they were decisively defeated at the Battle of Poitiers (also known as Battle of Tours and, by Arab sources, as Battle of the Palace of the Martyrs) by the Frankish general, Charles Martel (686-741), grandfather of the great Emperor Charlemagne.
This was seen as a Christian victory and, after Poitiers, there were no further attacks on Western Europe. The Crusades came much later.
The proximate causes of the First Crusade (1096-1099) include the defeat of the Byzantine Emperor Alexius Comnenus (1056-1118), who was crowned in 1081 and ruled until his death. His armies met the Muslim Seljuk Turks at the Battle of Manzikert in 1071 and were defeated.
This placed the city of Constantinople at risk of conquest. So, the emperor requested that the West send knights to assist him – and he was prepared to pay.
Pope Urban II (1044-1099) preached the Crusade at the Council of Clermont in 1095. He argued that the Turks and Arabs attacked Christian territories and had "killed and captured many, and have destroyed the churches and devastated the empire".
All who die by the way, whether by land or by sea, or in battle against the pagans, shall have immediate remission of sins. This I grant them through the power of God with which I am invested.
This was recorded by a monk called Fulcher of Chartres, who wrote a chronicle of the First Crusade.
Thousands answered the pope’s call and the First Crusade conquered Jerusalem in 1099. But the Crusaders' presence in the Middle East was short-lived and the port city of Ruad, the last Christian possession, was lost in 1302/3.
Many later conflicts that were called Crusades were not actions against Muslim armies at all. The Fourth Crusade (1202-1204), for instance, was a Venetian Catholic army, which besieged Constantinople. Catholic Christians attacked Orthodox Christians, then looted the city, taking its treasures back to Venice.
Islam was not a factor in the Albigensian Crusade of 1209-1229, either. In that instance, Pope Innocent III (1160/1-1216) used the language of war against the infidel (literally "unfaithful", meaning those without true religion) against heretics in the south of France. So, "right-thinking" Christians killed "deviant" Christians.
It wasn't all intermittent fighting. There were also periods of peace and productive relationships between Christian and Muslim rulers in the Middle Ages.
For instance, Charlemagne (742-814) (also know as Charles the Great or Charles I), who united most of Western Europe during the early part of the Middle Ages, sent gifts to Harun al-Rashid (763-809), the Caliph of Baghdad. In return, he received diplomatic presents such as a chess set, an elaborate clepsydra (water clock) and an elephant.
In Spain, the culture from the early eighth century to the late 15th was known as "la Convicencia" (the co-existence), as Jews, Christians and Muslims lived in relative peace (though the level of harmony has been exaggerated). And there was an exchange of ideas in fields including mathematics, medicine and philosophy.
The Christian kingdoms of the north gradually reconquered Al-Andalus. And, in 1492, King Ferdinand (1452-1516) and Queen Isabella (1451-1504) reclaimed Granada and expelled the Jews and Muslims from Spain, or forced them to convert to Christianity.
Clearly, to speak of an "us versus them" mentality, or to frame current geopolitical conflicts as "crusades" of Christians against Muslims, or vice versa, is to misunderstand – and misuse – history.
Modern Westerners would find medieval Crusader knights as unappealing as they do Islamic State.
It's a simplistic thing to say that 'Islamic State is neither Islamic nor a state', but there's an element of truth in it.
And it's impossible to miss the fact that the immediate entry into heaven Pope Urban promised to Christian soldiers who died in battle against the infidel Muslims is conceptually identical to the martyrdom ideology of contemporary jihadists.
Reality is more complex – and more interesting – than the simple continuation of a historical struggle against the same enemy. Muslims conquered Christian territories, yes, but Christians engaged in reconquest.
There were forced conversions to both Islam and Christianity, and – very importantly – actual governments and monarchs were involved. It's a simplistic thing to say that "Islamic State is neither Islamic nor a state", but there's an element of truth in it.
The most important reason we should resist the lure of the crusade tag to any fight against jihadists is that groups like Islamic State want the West to think like that.
It justified the Paris bomb attacks of November 2015 as attacks against "the Crusader nation of France". Osama bin Laden used the same reasoning after the September 11 attacks.
By adopting the role of Crusaders, Western nations play into Islamic State's hands. It's how these jihadists want the West to understand itself – as implacably opposed to Islam. But it's not, and it never has been.