It was Fallmerayer's opinion that the Slavonic invasions of the sixth century created a situation in which the Byzantine Empire, without actually having lost a single province, could consider as its subjects only the population of the seacoast provinces and fortified cities. The appearance of the Avars in Europe was an epoch-making event in the history of Greece because they brought with them the Slavs and spurred them on to conquer the sacred soil of Hellas and the Peloponnesus.
Fallmerayer based his theory primarily on the data found in the writings of the church historian of the late sixth century, Evagrius, who wrote: The Avars twice made an inroad as far as the Long Wall and captured Singidunum (Belgrade), Anchialus, and all of Greece, with other towns and fortresses, laying everything waste with fire and sword, while the greater part of the forces were engaged in the East. It was this mention of all of Greece in Evagrius that gave Fallmerayer a basis for speaking of the extermination of the Greek nation in the Peloponnesus. The Avars of Evagrius did not confuse this German scholar, for at that period the Avars attacked the Byzantine Empire conjointly with the Slavs. This particular invasion which Fallmerayer referred to the year 589, did not exterminate the Greeks completely. The final blow to the Greek population came, as Fallmerayer believed, with the importation of the plague from Italy in the year 746. Reference to this is found in the famous quotation from the imperial writer of the tenth century, Constantine Porphyrogenitus, who remarked that after this terrible plague the entire land was slavonized and became barbarian. The year when Emperor Constantine Copronymus died (775) Fallmerayer estimated, may be considered the final date when the desolate land became once more, and at this time completely, filled with Slavs, who gradually covered Greece with their new cities, towns, and villages.