The kingdom of the Franks was the only one of the German successor states that proved long-lasting. For several centuries, the Franks occupied a small area in northwestern Europe in present-day Belgium. Then, under the energetic monarch Clovis (481–511), who founded the Merovingian dynasty of Frankish kings, the Franks embarked on a period of expansion, conquering virtually all of France and a large part of western Germany. Breaking with Germanic custom, Clovis subjected the important nobles and warriors of his tribe to his authority. By doing so, he succeeded in enhancing his royal prestige and ensured that no other Frank would be strong enough to seize power from him. He also converted to Roman Christianity. With the backing of the church, he was able to further centralize his power as well as obtain a valuable ally in its leader, the pope.

For more than a century after his...