Pope Urban II brought us the First Crusade in 1095 AD after the Turks invaded Anatolia.
Pope Urban II ex Wiki
QUOTE
Urban II's crusading movement took its first public shape at the Council of Piacenza, where, in March 1095, Urban II received an ambassador from the Byzantine Emperor Alexios I Komnenos (1081–1118) asking for help against Muslim Turks, who had taken over most of formerly Byzantine Anatolia. A great council met, attended by numerous Italian, Burgundian, and French bishops in such vast numbers it had to be held in the open air outside the city. At the Council of Clermont held in November of the same year, Urban II's sermon proved highly effective, as he summoned the attending nobility and the people to wrestle the Holy Land and the eastern churches generally from the Seljuk Turks
UNQUOTE
Population growth triggered it off, or did it? Islam has been an enemy for a long time. It still is.