Crusades were a series of religiously-sanctioned military campaigns waged by much of Latin Christian Europe, particularly the Franks of France and the Holy Roman Empire. The specific crusades to restore Christian control of the Holy Land were fought over a period of nearly 200 years, between 1095 and 1291. Crusaders took vows and were granted penance for past sins, often called an indulgence. The immediate cause of the First Crusade was the Byzantine emperor Alexios I's appeal to Pope Urban II for mercenaries to help him resist Muslim advances into territory of the Byzantine Empire.