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.
In the reign of Justinian I (r. 527–565), 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 c. 615, 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.
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.
The Rhynchinoi, along with a Slavic tribe living in the neighbouring Strymon valley, the Strymonitai, were greatly upset by Perboundos' arrest. At their request, a joint delegation including Thessalonian envoys went to Constantinople to intercede on his behalf—a unique event, according to Byzantinist Paul Lemerle, that illustrates the surprisingly close and amicable relationship between the Byzantine city and its "barbarian" neighbours. The phrasing of the Miracles makes clear that Perboundos was guilty of some transgression, since the embassy was sent to request clemency but not exoneration. The emperor, who was in the midst of extensive preparations for a war with the Arabs, promised to set Perboundos free once the war was over. The envoys were apparently satisfied with this, and returned home; the agitation among the Slavs subsided for the moment. Subsequent events proved the eparch's suspicions well-founded. Perboundos managed to escape with the aid of an imperial translator charged with affairs concerning the Slavic tribes. A large-scale manhunt was launched against Perboundos, and fears of an imminent Slavic move on Thessalonica led the emperor to send a swift dromon to warn the city and instruct its leaders to take precautions and stockpile food in case of a siege. After forty days Perboundos was found hiding on the translator's estate near Bizye. The translator was executed, but Perboundos was returned to confinement in Constantinople, as before. After another failed attempt to escape, he publicly proclaimed his intention of raising all the Slavic tribes in revolt and seizing Thessalonica. Following his confession, he was executed.
On receiving news of Perboundos' execution, the Rhynchinoi rose in revolt, soon joined by the Strymonitai and another neighbouring tribe, the Sagoudatai. Many other Slavic tribes, however, did not join the revolt, and some, like the Belegezitai, proved willing to assist the Roman side.
Blockade and famine in Thessalonica
This Slavic league blockaded Thessalonica by land and raided its environs, with each tribe being assigned a specific area: the Strymonitai attacked from the east and north, the Rhynchinoi from the south, and the Sagoudatai from the west. Three or four raids were launched each day, both on land and at sea, for two years; all livestock was carried off, agriculture ceased, and maritime traffic was stopped. Anyone who ventured from the city walls was likely to be killed or captured. The historian Florin Curta comments that the Slavs "appear as better organized than in any of the preceding sieges, with an army of special units of archers and warriors armed with slings, spears, shields, and swords".
The city could expect little assistance from the emperor, who in the face of the Arab threat could not spare any troops. The situation was made worse by the city's authorities, who allowed the grain hoarded in the granaries, following the emperor's instructions, to be sold to foreign ships in the harbour, at a rate of a nomisma for seven modii, just one day before the start of the blockade. The anonymous author of the Miracles is highly critical of the commercial and civic elites for their greed and short-sightedness, which led to the rapid onset of famine inside the city. Exacerbated by a lack of water, the famine caused great suffering among the inhabitants, described at length in the text of the Miracles.
The situation became so bad that many Thessalonians defected to the besiegers, who in turn, fearful of so many Byzantines among them sold them on as slaves to other Slavic tribes of the Balkan interior; and only after some of these slaves escaped, bringing word of their sufferings to Thessalonica, did the defections cease. In the same context, but in passing, the author mentions the betrayal of a part of the Slavs to the north of the city, who, while appearing to trade with the city, slaughtered "the flower of our most valorous fellow citizens". The exact meaning of this passage is unclear; it may indicate a failed military operation by the besieged, or the massacre of a group of defectors who tried to return to the city, but it also indicates that at least a part of the besiegers (probably, based on their location, belonging to the Strymonitai) maintained relations with the city, and that the blockade was not entirely impenetrable.
Arrival of the Byzantine squadron and the great Slavic assault
Some relief was provided by the arrival of a squadron of ten armed transports, all that the emperor could spare, as he was engaged "in the other war" with the Arabs. However, according to the author of the Miracles, the sailors took advantage of the Thessalonians, and sold them the grain they had brought at highly inflated prices, while the authorities used them as manpower to seek out any hidden caches of grain in the city. The new arrivals were not sufficient to prevent the Slavs from operating freely in the city's environs; anyone who ventured outside the city walls, by land or sea, seeking food was in danger of attack. As a result, an assembly of the citizens and the local council decided to send the ten vessels, along with whatever watercraft could be found in the city, manned by the most vigorous citizens, to obtain food from the Belegezitai, who were living at the shores of the Pagasetic Gulf in Thessaly.