The Slavs are one of the indigenous peoples of Europe. Apparently the land of origin of the Slavs is the present-day territory of Poland, Belarus and Ukraine. The great migration of the peoples (4th-6th century AD) scattered the Slav tribes and formed three large groups - Western, Eastern and Southern Slavs.

In the middle of the 5th century the Southern Slavs inhabited a large territory to the North of the Danube River in the present lands of Hungary and Romania. In the late 5th and early 6th centuries the Southern Slavs, attracted by the Byzantium richness began to invade into the empire. More and more Slav tribes crossed the Danube (the North border of the Byzantine empire) and settled in the lands of Moesia, Thrace and Macedonia - the three “classical” regions comprising the future ethnic area of the Bulgarian people. In the early 7th century the Slavs had already colonized almost the whole peninsula, getting as far as Peloponnesus.

Engaged in crippling wars with Persians and Arabs the Byzantine empire had completely lost control over its European realms. The territory of the Balkans to the South of the Danube was still nominally Byzantine but the imperial power was represented only in several of the city centers which stood out as isolated islands among the Slav sea.

From the middle of the 7th century, extricated from its solicitude in Asia Minor, Byzantium began reconquering the Balkan Peninsula. The disunited Slav tribes in Greece, Macedonia and Thrace were brought under the sway of the Byzantine empire.

With a view to resisting the Byzantine reconquest, seven Slav tribes inhabiting Moesia (the area between the Danube and the Balkan Mountains), entered into a military union but its chances to counteract efficiently the mighty empire were minimal. However the appearance of a new military force - Bulgarian troops changed thoroughly the situation on the Balkans.