Greece became part of the Byzantine Empire when, in the third century, Diocletian divided the Roman Empire in two. The succession of emperors with authority over Greece is covered under Constantinople. By the Arthurian period, the western empire had fallen to the Vandals and the eastern empire was under attack by other barbarians. Greece was invaded by Huns, Slavs, and Goths. Arthurian legend fails to recognize this history.
In Culhwch and Olwen, Arthur has conquered Greece along with most of western Europe. Geoffrey of Monmouth has a king of Greece, Epistrophus, allying with Lucius of Rome in a war against Arthur. Chrétien de Troyes correctly makes it part of the empire of Constantinople and gives its succession of emperors as Alexander, Alis, and Cliges. The Post-Vulgate Queste del Saint Graal tells us that Erec was a descendant of Greeks; his grandfather, Canan, had been the King of Saloliqui. Finally, in Floriant et Florete, Greece is ruled by Emperor Filimenis and is later inherited by Sir Floriant.