In 580, however, the Avar khagan, Bayan I, marched with his men to the right bank of the Sava river across from Sirmium, and began construction of a bridge to cross it.

The city at the time was largely undefended and unprepared to withstand a siege, as most of the Byzantine forces were engaged in the east against Sassanid Persia.

Tiberius managed to send in a few officers from Dalmatia to oversee the city's defence, while the envoy Theognis tried unsuccessfully to treat with Bayan.

The Avars indeed spared the population, but took their possessions and 240,000 solidi from the Emperor, as arrears of the tribute owed over three years.

In the aftermath of the siege, many survivors fled to Salona, as evident form funerary inscriptions.