The Byzantines, focusing on their eastern border, where they faced the Sassanid Persians in a protracted war, were unable to maintain an effective defence of the region: following the fall of Sirmium in 582 and of Singidunum in the year after, the Balkans lay open to Avar raiding.

[4][5] After peace had been settled with the Persians in the East in 591, Byzantine emperor Maurice and his generals were able to drive back the Slavs and the Avars in a series of campaigns.

[6][7] The renewal of war with Persia meant the rapid and complete collapse of the Danube frontier in the first decades of the 7th century, as imperial forces were withdrawn to the East.

Phocas and his successor Heraclius bought off peace with the Avars through annual tributes, but the Slavs once again had a free hand in raiding the Balkans, and in 604, a force of 5,000 men suddenly attacked Thessalonica at night, but failed to scale the city walls.

[14][15] Although the technical sophistication of the besiegers was unprecedented, they were apparently unable to make full use of it due to inexperience: a siege tower collapsed and killed its crew, while the battering rams proved ineffective against the city walls.