The first series Harry Turtledove set in the Empire of Videssos took a Republican Roman legion and set it into this magical realm which was based on the seventh century Byzantine Empire which Rome would eventually become. Naturally enough, none of the legionnaires discovered that they were, in effect, seeing the future of their world.
In the two series and three short stories which have followed the original Videssos Cycle, Turtledove has examined the history of Videssos, continuously exploring earlier periods of Videssian history and her struggle with Makuran and Yezd. In the novel Bridge of the Separator, Turtledove examines Rhavas's origin as well as the beginnings of Kubrat and how it separated from Videssos. Since much of the cycle is loosely (and sometimes not so loosely) based on Byzantine/Persian history, the avid reader may be able to glean some clues from the study of their histories
The geography of the Empire of Videssos--and the rest of the fictional world in which Videssos is placed--is based upon the geography of the Mediterranean, Europe, and parts of Asia compressed and reversed so what lies to the East in our world lies to the West in Videssos and vice versa.
Most kingdoms and peoples correspond with real ones in our world and the history of Videssos parallels that of the Roman Empire. The Khamorth invasion of Videssos' northern and eastern provinces and the fall of Skopentzana parallels the barbarian invasions of the Western Roman Empire and the fall of Rome. The successor states that arose from the Khamorth invasion correspond to states and peoples in western Europe who--like the people of Khatrish, Thatagush, and Kubrat did--developed a religion based on but separate from the rest of the empire.