 Complete the following phrase:  In 680 CE, Bulgars crossed the Danube and invaded lands now part of modern-day Bulgaria, which was at that time largely inhabited by Slavic farmers. The Bulgar army was led by Asparukh, who according to Byzantine chroniclers was one of the five sons of Kubrat, the Onogur chief who revolted against his Avar rulers and succeeded in uniting the various Bulgar groups living north of the Black Sea. When Asparukh and his warriors entered the region south of the Danube, the Byzantine Emperor Constantine IV was upset and led an army to prevent the Bulgars from remaining there. The Byzantine expedition against the Bulgars 