Excerpts from Books and Wikipedia
"He continued his father's policy towards the papacy and became its protector, removing the Lombards from power in northern Italy and leading an incursion into Muslim Spain. He campaigned against the Saxons to his east, Christianizing them upon penalty of death and leading to events such as the Massacre of Verden." [Wikipedia] "Charlemagne and his successors, the Carolingians, presided over a short-lived period of renewed security, but their empire crumbled under the weight of succession disputes and repeated invasions by Scandinavian Vikings and eastern European Magyars." [Asbridge: The Crusades, p. 6] "Under the guidance of his personal tutor Alcuin, a British churchman whom Charlemagne had recruited to teach his sons, the king of the Franks developed a stronger and stronger sense of mission. His conquests were, in his own eyes, forceful evangelism: bringing the Gospel to stubborn unbelievers who needed to be saved not just from their sins but from their own unwillingness to hear." [Bauer: Medieval World, p. 389]