In the VIIth century, Moesia occupied an extremely important strategic position in the struggle between the Slavs and Byzantium. It was a bridge that connected the Slavs south of the Balkan Mountains to their tribesmen north of the Danube.
The invasions of the Slavic tribes across the Balkan Mountains were a serious concern for Byzantium. The empire made great efforts to consolidate and restore its power. The most vulnerable front for Moesia was towards the East : the Black Sea and Dobrudja. That is why the Byzantines built many fortresses and tried to make Dobrudja a base for their incursions on Slavic soil. The possibility of the Byzantine troops moving freely by sea towards their territory put the Slavs at a disadvantage.
The restoration of Byzantine rule over the northwestern part of the Balkan Peninsula seriously threatened the Slavic tribes both in Moesia and in the lands north of the Danube. The Eastern Roman empire, which also conquered the central part of the Balkan Peninsula, threatened Slavic tribes in Moesia from the south along the Balkan Mountains and all the way to the river Timok. Therefore, in order to fight against the empire, all the Slavic tribes between the Danube and the Balkan Range, as well as the tribes north of the river, had to get organised.
The information regarding the creation of this tribal union in the Danube region is contained in the works of the chroniclers Theophanes and Nicephorus (IXth century). The limitations of the historical sources also led to different ideas about the size and location of the Slavic tribal union.
Theophanes mentions seven Slavic tribes in addition to the northern ones, and Nicephorus speaks broadly of the Slavic tribes, without specifying their numbers.
The seven Slavic tribes were not under the rule of Byzantium and were not in its territory in the northeastern part of the Balkan Peninsula. The tribal union of the seven Slavic tribes is usually set up by Moesia. However, the sources explicitly state that its southern neighbours were the Byzantines, and to the west and northwest - the Avars. Therefore, many researchers believe that it was a Slavic tribal union in the Danube region, which extended on both sides of the Danube, from the Balkan Mountains to the Carpathians. These seven tribes may have included some of the Timochani, Moravian, Branichevci, and Obodrite tribes, whose names are not mentioned by the chroniclers.
The tribal union in the Danube region was an alliance of seven Slavic tribes in Moesia and Dacia, created in order to fight Byzantium and the Avars. The same way of life of the Slavic population on both sides of the Danube during the VIIth century also determined its territorial scope - from the Carpathians to the Balkan range. The main thing that united the tribes was the struggle against a foreign external danger, which is why political unions played a major role in the unification .
Under the new union, each Slavic tribe retained to a great extent its independence, governed by its own prince, and the territory occupied by it formed a separate principality, known by the name of Sclavinia.