On Christmas morning in the year 800, Pope Leo III placed the crown of imperial Rome on the brow of a Germanic king named Karl--a gesture that enabled the man later hailed as Charlemagne to claim his empire and forever shape the destiny of Europe. Becoming Charlemagne tells the story of the international power struggle that led to this world-changing event, illuminating an era that has long been overshadowed by myth.

For 1,200 years, the deeds of Charlemagne inspired kings and crusaders, the conquests of Napoleon and Hitler, and the optimistic architects of the European...