Thus occurred the spectacle of the leader of the Christian church, on Christmas Day, in the great Basilica of St. Peter, placing a crown on the head of the king -- and prostrating himself in a Roman ritual resembling an act of emperor worship. It didn't matter that Charles had multiple wives and mistresses. Nor did it trouble the pontiff that he had a reputation for ruthlessness, earned during his wars against the Saxons, a Germanic tribe of pagan worshipers. In 782, in the Massacre of Verden, Charles ordered the execution of 4,500 prisoners, apparently for their refusal to convert...