 Refused, they began pillaging the northern and eastern Balkans, which only ended after the Avars were pushed back by the Byzantines from 597 to 602.After capturing Sirmium, the Avars began to rapidly encroach into the Balkans.Their rapid spread was facilitated by the ongoing Byzantine–Sasanian War of 572–591, which left the Byzantine garrisons on the Danube frontier under-manned and underpaid.