The Siege of Sirmium in 580–582 was a decisive event in the history of the Balkans. The fall of the city to the Avars deprived the Byzantine Empire of its major stronghold on the northwestern Danube, opening the path for devastating incursions by the Avars and their Slavic allies into the Balkans.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.