I mean, if the Crusades had been brought on by the lightning-fast rise of the Islamic Empire and a desire to keep in Christian hands the land of Jesus, then they would have started in the 8th century. But early Islamic dynasties like the Umayyads and the Abbasids were perfectly happy with Christians and Jews living among them, as long as they paid a tax. And plus, the Christian pilgrimage business was awesome for the Islamic Empire's economy. But then a new group of Muslims, the Seljuk Turks, moved into the region, and they sacked the holy cities and made it much more difficult for Christians to make their pilgrimages, and while they quickly realized their mistake, it was already too late. The Byzantines, who'd had their literal asses kicked at the Battle of Manzikert in 1071, felt the threat and called upon the West for help. So the First Official Crusade began with a call to arms by Pope Urban II in 1095 CE. This was partly because Urban wanted to unite Europe, and he'd figured out the lesson the rest of us learned from alien invasion movies.