 This included Sirmium, which had been recently reconquered by the Byzantines from the Gepids, and would serve as the first cause of conflict between the Avars and the Byzantines.The Avars were heavily dependent upon the skills and labor of their subject peoples for both siege warfare and logistics.Subject peoples, such as the early Slavs and the Huns, had long traditions of engineering and craftsmanship, such as the building of boats and bridges, and the use of rams, tortoise formations, and artillery in sieges.