While the Byzantines were occupied with the Persians, the Avars and Slavs poured into the Balkans, capturing several Byzantine cities. Because of the need to defend against these incursions, the Byzantines could not afford to use all their forces against the Persians. Heraclius sent an envoy to the Avar Khagan, saying that the Byzantines would pay a tribute in return for the Avars withdrawing north of the Danube. The Khagan replied by asking for a meeting on 5 June 623, at Heraclea in Thrace, where the Avar army was located; Heraclius agreed to this meeting, coming with his royal court. The Khagan, however, put horsemen en route to Heraclea to ambush and capture Heraclius, so they could hold him for ransom.

Heraclius was fortunately warned in time and managed to escape, chased by the Avars all the way to Constantinople. However, many members of his court, as well as an alleged 70,000 Thracian peasants who came to see their Emperor, were captured and killed by the Khagan's men. Despite this treachery, Heraclius was forced to give the Avars a subsidy of 200,000 solidi along with his illegitimate son John Athalarichos, his nephew Stephen, and the illegitimate son of the patrician Bonus as hostages in return for peace. This left him more able to focus his war effort completely on the Persians.