Justin ceased making payments to the Avars, which had been implemented by his predecessor, Justinian. The Avars almost immediately launched an attack on Sirmium in 568, but were repulsed. The Avars withdrew their troops back to their own territory, but allegedly sent 10,000 Kotrigur Huns, a people who like the Avars had been forced into the Carpathians by the Turkic Khaganate, to invade the Byzantine province of Dalmatia. They then began a period of consolidation, during which the Byzantines paid them 80,000 gold solidi a year.

Except for a raid on Sirmium in 574, they did not threaten Byzantine territory until 579, after Tiberius II stopped the payments. The Avars retaliated with another siege of Sirmium. The city fell in c. 581, or possibly 582. After the capture of Sirmium, the Avars demanded 100,000 solidi a year. Refused, they began pillaging the northern and eastern Balkans, which only ended after the Avars were pushed back by the Byzantines from 597 to 602.