The First Crusade was initiated by Pope Urban II during the Council of Clermont. Responding to a plea for help from the Byzantine Emperor Alexios I Komnenos, the Pope called for a 'holy war' to reclaim Jerusalem and the Holy Land from Muslim control. He famously promised the remission of sins to all who participated. This event triggered a massive movement of knights, peasants, and clergy across Europe toward the Levant. In 1099, the Crusaders successfully captured Jerusalem, establishing several Crusader States. This event had profound long-term consequences, including the intensification of religious friction between Christianity and Islam, the expansion of trade between East and West, and the introduction of Eastern knowledge and technology to Europe. It marked the beginning of a centuries-long era of military-religious expeditions that reshaped the Mediterranean world.