" Charlemagne, however, was predictably furious. He had grown up with the filioque; if the East refused to accept it, the East was wrong. And who cared about the East anyway? He was the emperor now; the pope should nail his colors firmly to the Western mast and leave the heretics in Constantinople to their own devices. When Leo ordered him to remove the word from his liturgies, he took no action and sent no reply; and when, in 813, he decided to make his son Louis co-emperor, he pointedly failed to invite the pope to perform the ceremony. "

― John Julius Norwich , Absolute Monarchs: A History of the Papacy