Why is there a tension between the Muslim and Christian worlds at the start of the Crusades?

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It’s often said that winners dictate history. Not so for the medieval holy wars called the Crusades.

Muslim forces ultimately expelled the European Christians who invaded the eastern Mediterranean repeatedly in the 12th and 13th centuries—and thwarted their effort to regain control of sacred Holy Land sites such as Jerusalem. Still, most histories of the Crusades offer a largely one-sided view, drawn originally from European medieval chronicles, then filtered through 18th and 19th-century Western scholars.
But how did Muslims at the time view the invasions? (Not always so contentiously, it turns out.) And what did they think of the European interlopers? (One common cliché: “unwashed barbarians.”) For a nuanced view of the medieval Muslim world, HISTORY talked with two prominent scholars: Paul M. Cobb, professor of Islamic History at the University of Pennsylvania, author of Race for Paradise: An Islamic History of the Crusades, and Suleiman A. Mourad, a professor of religion at Smith College and author of The Mosaic of Islam.
HISTORY: Broadly speaking, how do Islamic perspectives on the Crusades differ from those of the Christian sources from Western Europe?
Suleiman Mourad: If we wrote the history of the Crusades based on Islamic narratives, it would be a completely different story altogether. There were no doubt wars and bloodshed, but that wasn’t the only or dominant story. There was also coexistence, political compromise, trade, scientific exchange, love. We have poetry and chronicles with evidence of mixed marriages.
Do Muslim perspectives match Western ones in terms of chronology and geography?
Paul Cobb: Chronologically, Muslim sources differ from the Christians because they don’t recognize the Crusades. They recognize the events we call the Crusades today simply as another wave of Frankish aggression on the Muslim world. (I use “Franks” or “Frankish” to refer to western Christians.) For them, the Crusades didn’t begin in Clermont with Pope Urban’s 1095 speech [rallying crusaders], as most historians say, but rather decades earlier. By 1060 Christians were not only nibbling at the edges of the Islamic world, but were actually gaining territory in Sicily and Spain. And whereas most Western historians recognize the 1291 fall of Acre as the end of the main Crusades, Muslim historians don’t see the end of the Frankish threat until, I would say, the mid-15th century, when Ottoman armies conquer Constantinople.
SM: To say the Crusades started in Clermont in 1095 and ended at Acre in 1291, we are fooling ourselves. History is not that clean cut. What came before and after reflected a lot of continuity and not abrupt change.