The siege of Thessalonica in 676–678 was an attempt by the local Slavic tribes to capture the Byzantine city of Thessalonica, taking advantage of the preoccupation of the Byzantine Empire with the repulsion of the First Arab Siege of Constantinople. The events of the siege are described in the second book of the Miracles of Saint Demetrius.
Background
In the reign of Justinian I (), Slavic tribes (Sclaveni) had already appeared on the Danube frontier of the Byzantine Empire. Over the next few decades, they raided into Thrace and Illyricum, while at times serving as mercenaries in the Byzantine army. From the 560s, the Slav communities came under the control of the newly established Avar Khaganate. Raids became larger and resulted in permanent settlement, especially as the Avars were able to capture fortified cities, leading to loss of imperial control over the surrounding areas. While the Byzantines were preoccupied in the East against the Persians, the 580s saw ever deeper and more destructive raids in the Balkans, even into southern Greece. The same period saw the start of large-scale Slavic settlement in the Balkan hinterland. After making peace with Persia, Emperor Maurice was able to launch a series of counterattacks that drove the Avars and their Slavic allies back, but the respite was short-lived: following the usurpation of Phocas in 602, and the start of another, and even more catastrophic, war with Persia, the Balkans were left nearly defenseless, and the Danube frontier collapsed once more, overrun by the Avars and Slavic tribes, who settled across the region.
By the 610s, the city of Thessalonica was surrounded by large Slavic settlements, being reduced to itself within its wall, according to historian John Van Antwerp Fine, to "virtually a Roman island in a Slavic sea". The first book of the Miracles of Saint Demetrius records the attempts by the Slavs to capture the city in that time, first an unsuccessful attack by the Slavic leader Chatzon in , followed by an unsuccessful siege by the Avars and Slavs in 617. By the middle of the 7th century, more cohesive Slavic coalitions (Sclaviniae) had been established in the former Roman Balkans. The only imperial reaction came in 658, when Emperor Constans II campaigned in Thrace, brought many Sclaviniae under imperial control, and relocated many Slavs to Asia Minor.
Immediate cause of the siege
The second book of the Miracles of Saint Demetrius names Perboundos, the "king of the Rhynchinoi", as a powerful ruler, who was sufficiently assimilated to be able to speak Greek, had relations with Thessalonica to the point of maintaining a residence there, and even dressed in the Byzantine style. According to the Miracles, the peace existing between the Slavs and the Byzantines ended when the—unnamed—Byzantine eparch of Thessalonica was informed that Perboundos planned to move against the city. The eparch reported this to the Byzantine emperor, who ordered his arrest. After informing the city elders, the eparch had Perboundos arrested during his stay in the city, put in irons and sent to the Byzantine capital, Constantinople.