The Avars decided to take advantage of the lack of troops in the Balkans by besieging Sirmium which falls in 579 CE. At the same time, the Slavs began to migrate into Thrace, Macedonia and Greece, which Tiberius was unable to halt as the Persians refused to agree to a peace in the east, which remained the emperor's main priority. By 582, with no apparent end to the Persian war in sight, Tiberius was forced to come to terms with the Avars, and he agreed to pay an indemnity and to hand over the vital city of Sirmium, which the Avars then looted. The migration of the Slavs continued, with their incursions reaching as far south as Athens.

The Slavic migrations to the Balkans have taken place since the mid-6th century and first decades of the 7th century in Early Middle Ages. The rapid demographic spread of the Slavs was followed by a population exchange, mixing and language shift to and from Slavic. There was no single reason for the Slavic migration that would apply to most of this area to became Slavic-speaking. The settlement was facilitated by the substantial fall of the Balkan population during the Plague of Justinian. Another reason was the Late Antique Little Ice Age from 536 to around 660 CE and the series of wars between the Sasanian Empire and the Avar Khaganate against the Eastern Roman Empire. The backbone of the Avar Khaganate consisted of Slavic tribes.