Sirmium, which for much of the 6th century had been controlled by the Goths and then the Gepids, had come under Byzantine control in 567. The Avars appeared along the Danube at about the same time. They launched a first attack on Sirmium in 568, but were seen off by the local governor, Bonus. The Byzantines secured peace with the Avars through the payment of an annual tribute, which by 578 had risen to some 80,000 solidi. 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. The Byzantine emperor Tiberius II tried to