The Slavonic advance and the origin of the Bulgarian kingdom
From the second half of the sixth century the Slavs not only continually attacked and pillaged the Balkan possessions of the Byzantine Empire, but they reached as far as the Hellespont, Thessalonica, southern Greece, and the shores of the Adriatic Sea, and settled there in large numbers. The Avaro-Slavonic attack on the capital occurred in the year 626, during the reign of Heraclius; in the period of the Heraclian dynasty the Slavs persistently advanced into the peninsula and began to populate it very densely. Thessalonica became surrounded by Slavonic tribes and found it difficult to seek protection against their attacks even within its strong city walls.
In their vessels the Slavs descended to the Aegean Sea, attacking the Byzantine fleet and frequently cutting off the supply of provisions to the capital. The emperor Constans II was forced to undertake a campaign against Sclavinia. From this time dates the migration of large masses of Slavs to Asia Minor and Syria. Under Justinian II a horde of Slavs numbering no less than 80,000, according to V. I. Lamansky, were transported to Opsikion, one of the themes of Asia Minor. One part of them (about 30,000) was mobilized by the Emperor and later took part in the struggle with the Arabs, during which they deserted the Emperor and sided with the Muhammedans. For this terrible offense the remaining Slavs of Opsikion were subjected to formidable massacres. A seal of the Slavonic military colony of Bithynia, a province in the theme of Opsikion, has survived from this period. It is a monument of great value, a new fragment of Slavonic tribal history, which affords a ray of light in the twilight of the great migrations, as B. A. Panchenko, who published and interpreted this seal, declared. Beginning with the seventh century, the problem of Slavonic settlements in Asia Minor assumes a very profound significance.