Dale T. Irvin & Scott W. Sunquist, History of the World Christian Movement, Vol 1: Earliest Christianity to 1453. Maryknoll, New York: Orbis Books, 2006, pp. 234-41.
Charlemagne (reigned from 768-814 C.E.)
Forcing entire peoples to either undergo Christian baptism or face execution and establishing capital punishment for worship of any traditional gods, failing to baptize one’s children, cremating the dead, or even eating meat during Lent, Charlemagne’s rule was excessively brutal (at one point he had four thousand Saxon prisoners of war executed) and was the first full-scale use of military force and violence to compel peoples to convert to Christianity.
Ruling from his capital Aachen (in modern Belgium), at the height of Charlemagne’s power, he ruled the majority of modern France, Germany, Austria, and Hungary (all the way to the Balkans) and eventually took on many of the trappings of Roman imperial identity. Even the Islamic caliph Harun al-Rashid (best remembered for his role in “1001 Arabian Nights”) responded to news of Charlemagne’s enthronement by sending a gift!
The rule of Charlemagne ushered a period known as the Carolingian Renaissance in the West in which Charlemagne encouraged schools and a steady flow of Latin texts into his capital, invited the best theologians in the West to come to his capital, and issued a series of General Directives intended to address a number of social reforms (for example, the legal status of marriage put an end to polygamy even in the aristocracy and forbade priests to marry, he enforced the exclusive use of the Rule of Benedict, secured the caliph’s permission to build a monastery in Jerusalem, endorsed and forced the filioque phrase to be used in the Latin versions of the Nicene Creed recited in the churches to the displeasure of the Pope Leo III, etc.).
Charlemagne’s legacy lived on beyond his kingdom and long after the Carolingian line of kings (which came to an end in 911 C.E.) and set a precedent, one that Christian rulers would emulate in the centuries to come.