Under the reign of Asparuh, Bulgaria expanded southwest after the Battle of Ongal and Danubian Bulgaria was created. The son and heir of Asparuh Tervel becomes ruler in the beginning of 8th century when the Byzantine emperor Justinian II asked Tervel for assistance in recovering his throne, for which Tervel received the region Zagore from the Empire and was paid large quantities of gold. He also received the Byzantine title "Caesar". After the reign of Tervel, there were frequent changes in the ruling houses, which lead to instability and political crisis.

Decades later, in 768, Telerig of the house Dulo, ruled Bulgaria. His military campaign against Constantine V in the year 774, proved to be unsuccessful. Under the reign of Krum (802–814) Bulgaria expanded vastly north-west and south, occupying the lands between the middle Danube and Moldova rivers, all of present-day Romania, Sofia in 809 and Adrianople in 813, and threatening Constantinople itself. Krum implemented law reform intending to reduce poverty and strengthen social ties in his vastly enlarged state.

During the reign of Khan Omurtag (814–831), the northwestern boundaries with the Frankish Empire were firmly settled along the middle Danube. A magnificent palace, pagan temples, ruler's residence, fortress, citadel, water mains and baths were built in the Bulgarian capital Pliska, mainly of stone and brick.

By the late 9th and early 10th centuries, Bulgaria extended to Epirus and Thessaly in the south, Bosnia in the west and controlled all of present-day Romania and eastern Hungary to the north reuniting with old roots. A Serbian state came into existence as a dependency of the Bulgarian Empire. Under Tsar Simeon I of Bulgaria (Simeon the Great), who was educated in Constantinople, Bulgaria became again a serious threat to the Byzantine Empire. His aggressive policy was aimed at displacing Byzantium as major partner of the nomadic polities in the area.

After Simeon's death, Bulgaria was weakened by external and internal wars with Croatians, Magyars, Pechenegs and Serbs and the spread of the Bogomil heresy.^[23] Two consecutive Rus' and Byzantine invasions resulted in the seizure of the capital Preslav by the Byzantine army in 971.^[24] Under Samuil, Bulgaria somewhat recovered from these attacks and managed to conquer Serbia and Duklja.^[25]

In 986, the Byzantine Emperor Basil II undertook a campaign to conquer Bulgaria. After a war lasting several decades he inflicted a decisive defeat upon the Bulgarians in 1014 and completed the campaign four years later. In 1018, after the death of the last Bulgarian Tsar - Ivan Vladislav, most of Bulgaria's nobility chose to join the Eastern Roman Empire.^[26] However, Bulgaria lost its independence and remained subject to Byzantium for more than a century and a half. With the collapse of the state, the Bulgarian church fell under the domination of Byzantine ecclesiastics who took control of the Ohrid Archbishopric.