The Slavic migrations to the Balkans began in the mid-6th century and first decades of the 7th century in the Early Middle Ages. The rapid demographic spread of the Slavs was followed by a population exchange, mixing and language shift to and from Slavic. The settlement was facilitated by the substantial decrease of the Balkan population during the Plague of Justinian. Another reason was the Late Antique Little Ice Age from 536 to around 660 CE and the series of wars between the Sasanian Empire and the Avar Khaganate against the Eastern Roman Empire. The backbone of the Avar Khaganate consisted of Slavic tribes. After the failed siege of Constantinople in the summer of 626, they remained in the wider Balkan area after they had settled the Byzantine provinces south of the Sava and Danube rivers, from the Adriatic towards the 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 regain its lost territories, so it reconciled with the establishment of Sklavinias influence and created an alliance with them against the Avar and Bulgar Khaganates.