Slavic migrations to the BalkansBulgaria
The Slavs, of Indo-European origin, were first mentioned in written sources to inhabit the territories to the north of the Danube in the 5th century AD but most historians agree that they had arrived earlier. The Slavic incursions in the Balkans increased during the second half of Justinian I's reign and while these were initially pillaging raids, large-scale settlement began in the 570s and 580s.
Consumed in bitter wars with the Persian Sasanian Empire in the east, the Byzantines had few resources with which to confront the Slavs. The Slavs came in large numbers and the lack of political organisation made it very difficult to stop them because there was no political leader to defeat in battle and thereby force their retreat.