However, before the Bulgarians who settled in Scythia Minor allied with the Slavs and thus set the beginning of the state unification known as the First Bulgarian Kingdom, a remarkable event took place. It was the siege of Thessaloniki by the Slavic tribes. 
The reason for the rise of the Slavic tribes against Thessaloniki was the murder of the Slavic prince Prebund, fomented by Byzantium. The prince was preparing his people for an uprising against the empire. His death became the spark that ignited the fire of the largest siege of Thessaloniki by the Slavs, which lasted more than two years (675-677).
The siege of the flourishing Byzantine city began in 675 with the unification of the tribes of Rhynchines, Strymonites and Sagudates. For two years, on land and at sea, the Strymonites attacked the eastern and northern outskirts of the city, and the Rhynchines and Sagudates attacked the western and coastal areas.
The organisation of so many warriors and tribes for this campaign clearly shows that although Byzantium considered the Slavs to be barbarians who wreak havoc in the lands, they were actually well organised and cohesive, especially when they had to lead campaigns of this kind. The Slavs, who earned a lot of their skills about the construction of siege machines and equipment from the lands of Byzantium, showed great ingenuity in this siege and created deep problems for the empire.
The heavy and prolonged siege left Thessaloniki without food, and the population lived in constant fear. The desperate attempts of its inhabitants to provide for themselves and the constant shortage led to the flight of many from the besieged city. Others voluntarily surrendered to the Slavs themselves.
Although Constantinople was preoccupied with the conflict with the Arabs, the threat of the empire losing the important administrative and political center of Thessaloniki forced the emperor to send a significant number of ships loaded with troops and food to help defend it.
After the Slavs roamed around the city for two years and cut off virtually all roads to Thessaloniki in 677, they laid siege to the city itself. The Draguvites also joined the siege. In line, they placed "incendiary weapons and some devices made of pleated sticks, ladders as high as the sky, as well as stone throwers, other devices made of innumerable wooden tools and newly prepared throwing weapons against the doors." This is how an anonymous Byzantine author describes the siege in The Miracles of St. Demetrius of Thessaloniki.
The very attack of the Slavic tribes against the fortress walls of Thessaloniki was undertaken on July 25, 677. The attack began both on land and at sea. Some other Slavic tribes, not mentioned by name, also took part in the siege.
The siege of Thessaloniki unfolded as follows. The siege weapons were brought close to the fortress wall. The warships and galleys of the Slavs also approached the wall. An inspection started of all the places where an attack was to take place. On the second day, shouting battle cries, the Slavs stormed the city together with the war machines. Some of the Slavs advanced by land, while others crossed the water obstacles on pontoon bridges from ships and boats. Slavic infantry units included archers, shield bearers, light units, slingshots and war machine technicians. The city was shelled with numerous arrows and incendiary projectiles.
During the assault, the Slavs managed to burn one of the city gates by lighting a large fire in front of it and throwing a lot of wood into it. As they did so, their archers and spearmen fired against the walls, forcing the Byzantine soldiers to stand behind their shelters. However, the iron structure of the gate withstood, and the Eastern Roman Empire managed to put up a fierce resistance and prevent the attackers to enter the city.
The storming of the city lasted three days, with the Slavs concentrating their troops in different places. Finally, the ships returned to the aid of the city, which were sent to the Belegezites, bringing a lot of grain and legumes. The Slavs then withdrew some distance from the city, continuing their attacks and setting up ambushes. At the same time, they began to prepare new siege machines.
Despite the failure of the siege, the battle for Thessaloniki did not end. The Slavs continued their daily attacks, ambushed its surroundings and attacked the Byzantine ships transporting food from Constantinople to Thessaloniki. This they did "at the islands and the narrow sea, at the places near Parion and Proconesus" according to the unknown Byzantine chronicler and historian. These actions continued in the second half of 677 and throughout 678.
A turning point in Byzantine-Slavic relations occurred after 678, when the empire repelled the Arab attacks on Constantinople and could direct its forces against the Slavs on the Balkan Peninsula. The military actions of the Slavs had reached a wide scale. Now they not only threatened Thessaloniki and its surroundings, but threatened the order and peace in the empire itself.
The Byzantine campaign against the Slavs, as the chronology of events shows, took place in 679. Although the Slavs were preparing to meet the advancing Byzantine army in their gorges and fortifications along the Strimon River, they were defeated. Many of their leaders and prominent warriors perished. This crushing blow put an end to the Slavic attempts to capture Thessaloniki. At the same time, the defeat of the Slavic tribes played an important role in their consolidation with the Bulgarians of Khan Asparukh who settled in Scythia Minor. The Slavs turned to them for help. This led to political unification between Bulgarians and Slavs, which later became the solid foundation on which Danubian Bulgaria was built.
Although the Slavic siege of Thessaloniki failed, it showed that the empire was not as strong as it appeared from the outside. Taking advantage of the alliance with the Slavic tribes and providing them with protection with his cavalry, Khan Asparukh managed to move the centre of his country from the swampy and poor lands in the area around Ongal to the lands north of the Balkan Mountains. From this point of view, the Slavic siege of Thessaloniki can also be seen as a catalyst that lead to a faster and easier consolidation of the two communities - Slavic and Bulgarian.