The Byzantine Empire, based on the territory of the former Eastern Roman Empire, adopted Christianity under its first Emperor Constantine I and reached its greatest territorial extent under Justinian I in the mid 6th century. In 681 CE Bulgar tribes moving into the northeastern Balkans, defeated the Byzantine army, led by Constantine IV, and founded the First Bulgarian Empire. Relations with their near-neighbours, the Byzantines, were frequently fraught and the Byzantine emperors were always keen to exploit any internal instability in the Bulgaria to eliminate their rival. In the 8th century, despite internecine warfare, the Byzantines never managed to gain suzerainty over Bulgaria and in the 9th century the Byzantines could only look on as Bulgaria almost doubled in size, expanding both southwards and northwestwards. By the mid-9th century Bulgaria had become Christian and entered a golden age; it also represented a continued threat to the Byzantine Empire. Peace was intermittently achieved but by the beginning of the 11th century the two neighbours were once again at war, culminating in the decisive defeat of the Bulgarians at Kleidon in 1014. In 1018 Bulgaria was annexed to the Byzantine Empire and the first Bulgarian Empire came to an end.