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. The best way to get people to unite is to give them a common enemy. So Urban called on all the bickering knights and nobility of Europe, and he saideth unto his people, Let us go forth and help the Byzantines, because then maybe they will acknowledge my awesomeness and get rid of their stupid not-having-me-as-Pope thing, and while we're at it, let's liberate Jerusalem. I'm paraphrasing, by the way.