The first appearance of the Slavs in the Byzantine Empire can be dated no earlier than the 6th century. Throughout this century, beginning with the reign of Justinian, Slavs repeatedly invaded the Balkan possessions of the Byzantine Empire. Not until the reign of Maurice, however, did any Slavs settle in these territories. Between the years 579-587 there took place the irruption of several barbarian waves led by the Avars, but consisting mostlyof Slavs. The latter came in great numbers, and, as the troops of the Empire were engaged in the war with Persia, they roamed the country at will.
Slavs devastated Illyricum and Thrace, penetrated deep into Greece and the Peloponnesus, helped the Avars to take numerous cities, including Singidunum, Viminacium (Kostolac), Durostorum (Silistria), Marcianopolis, Anchialus, and Corinth, and in 586 laid siege to the city of Thessalonica, the first of a series of great sieges which that city was destined to undergo at their handss What is more, they came to stay.
"The Slavonians," wrote John of Ephesus in 584, "still encamp and dwell in the Roman territories and live in peace there, free from anxiety and fear, and lead captives and slay and burn." The counter offensive launched by Maurice after 591, following the successful termination of the Persian war, had the effect, on the whole, of checking the repeated incursions of the Avars, who then seem to have transferred their operations farther west beyond the limits of Byzantine territory. The treaty of peace which the Empire concluded with them in possibly in 601, fixed the Danube as the boundary line between the two powers, but left the way open for the Byzantines to cross that river and chastise any Slavs that might appear dangerous.80 There is no indication, however, that the Slavs who had penetrated into the Empire were forced to retire beyond the Danube, or that they did so of their own accord.
Settlement of the Slavs in the Balkan Peninsula occurred mainly in the....