The Slavic migrations to the Balkans have taken place since the mid-6th century and first decades of the 7th century in Early Middle Ages after a series of wars between the Sasanian Empire and the Avar Khaganate against the Eastern Roman Empire. The settlement was facilitated by the substantial fall of the Balkan population during the Plague of Justinian. The backbone of the Avar Khaganate consisted of Slavic tribes, which, after the failed siege of Constantinople in the summer of 626, remained in the wider Balkan area after they had settled the Byzantine provinces south of the river Sava and Danube, from the Adriatic toward Aegean up to the Black Sea. Exhausted by several factors and reduced to the coastal parts of the Balkans, Byzantium was not able to wage war on two fronts and to regai