Very important events took place in the Balkan peninsula after the death of Justinian, although unfortunately present knowledge of them is limited by the fragmentary material that appears in the sources. During Justinian's reign the Slavs frequently attacked the provinces of the Balkan peninsula, penetrating far into the south and threatening at times even the city of Thessalonica. These irruptions continued after Justinian's death. There were then large numbers of Slavs remaining in the Byzantine provinces, and they gradually occupied the peninsula. They were aided in their aggression by the Avars, a people of Turkish origin living at that time in Pannonia. The Slavs and Avars menaced the capital and the shores of the Sea of Marmara and the Aegean, and penetrated into Greece as far as the Peloponnesus. The rumor of these invasions spread to Egypt, where John, bishop of Nikiu, wrote in the seventh century, during the reign of the Emperor Phocas: It is recounted that the kings of this epoch had by means of the barbarians and the foreign nations and the Illyrians devastated Christian cities and carried off their inhabitants captive, and that no city escaped save Thessalonica only; for its walls were strong, and through the help of God the nations were unable to get possession of it. A German scholar of the early nineteenth century held the theory, discussed at length later, that at the end of the sixth century the Greeks were completely destroyed by the Slavs. Studies of the problem of Slavic settlement in the Balkan peninsula depend greatly upon the Acts of the martyr Demetrius, the protector of Thessalonica, one of the main Slavonic centers in the peninsula.
At the end of the sixth and the beginning of the seventh century the persistent southward movement of the Slavs and Avars, which Byzantine troops were unable to stop, produced a profound ethnographic change in the peninsula, since it became occupied largely by Slavonic settlers. The writers of this period were, in general, poorly acquainted with the northern tribes and they confuse the Slavs and Avars because they attacked the Empire jointly.