Sclaveni raided and settled the western Balkans in the 6th and 7th century. Jointly with the Antes, another Slavic group, they conducted intrusions south of the Danube and Sava rivers into the Balkans, and the territory of the Byzantine Empire ruled by Justinian I (527-565), who almost revived the Roman Empire. The arrival of the Avars in the Pannonian Plain in 567 pushed the proper invasion raids. The Slavs followed the Avars in their, mostly destructive enterprises, into the Byzantine territory. They destroyed and conquered one by one city and fortress which constituted the Danubian Limes, northern border of the empire, like Sirmium (582) and Singidunum (modern Belgrade, 584). In 584 and 586 the Slavs already besieged Thessaloniki, on the Aegean Sea, raided Dalmatia in 597 while the entire limes collapsed by 602.