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 ended disastrously and after defeating the imperial forces, Asparukh forced the nearby Slavic tribes to pay tribute to him, while leaving their tribal organization intact. As a result of this defeat, the empire was forced to sign a treaty recognizing the Bulgar state in 681. Despite the area having been for some time under the de facto control of various Slavic tribes, the emperors of Byzantium had kept up the fiction that the area was still imperial, as up to 681 no true state had replaced imperial rule. This was the first time that the Byzantine empire officially acknowledged another state in the Balkans.