 This led to them attempting to seize the city of Sirmium from Byzantium, which had previously retaken it from the Gepids, without success.Most of their future conflicts came as a result of raids by the Avars, or their subject Slavs, into the Balkan provinces of the Byzantine Empire.The Avars usually raided the Balkans when the Byzantine Empire was distracted elsewhere, typically in its frequent wars with the Sassanid Empire in the East.