First Bulgarian Empire

During the time of the late Roman Empire, the lands of present-day Bulgaria had been organised in several provinces - Scythia (Scythia Minor), Moesia (Upper and Lower), Thrace, Macedonia (First and Second), Dacia (Coastal and Inner, both situated south of Danube), Dardania, Rhodope and Hemimont, and had a mixed population of Thracians, Greeks and Dacians, most of whom spoke either Greek or a Latin-derived language known as Romance. Several consecutive waves of Slavic migration throughout the 6th and the early 7th century led to a dramatic change of the demographics of the region and its almost complete "Slavonisation".
Under the warrior Khan Krum (802-814), Bulgaria expanded northwest and southwards, occupying the lands between middle Danube and Moldova, the whole territory of present-day Romania, Sofia in 809 and Adrianople in 813, and threatening Constantinople itself. Khan Krum implemented law reform intending to reduce the poverty and to strengthen the 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 and magnificent palace, pagan temples, ruler's residence, fortress, citadel, water-main and bath were built in Bulgarian capital Pliska, mainly of stone and brick.
Under Boris I the Bulgarians became Christians, and the Ecumenical Patriarch agreed to allow an autonomous Bulgarian Archbishop at Pliska. Missionaries from Constantinople, Cyril and Methodius, devised the Glagolitic alphabet, which was adopted in the Bulgarian Empire around 886. The alphabet and the Old Bulgarian language gave rise to a rich literary and cultural activity centered around the Preslav and Ohrid Literary Schools, established by order of Boris I in 886. In the beginning of 10th century AD, a new alphabet - the Cyrillic alphabet - was developed on the basis of Greek and Glagolitic cursive at the Preslav Literary School. According to an alternative theory, the alphabet was devised at the Ohrid Literary School by Saint Climent of Ohrid, a Bulgarian scholar and disciple of Cyril and Methodius.
By the late 9th and the beginning of the 10th century, Bulgaria extended to Epirus and Thessaly in the south, Bosnia in the west and controlled the whole of present-day Romania and eastern Hungary to the north. A Serbian state came into existence as a dependency of the Bulgarian Empire. Under Tsar Simeon I (Simeon the Great), who was educated in Constantinople, Bulgaria became again a serious threat to the Byzantine Empire. Simeon hoped to take Constantinople and make himself Emperor of both Bulgarians and Greeks, and fought a series of wars with the Byzantines through his long reign (893-927). The war boundary towards the end of his rule reached Peloponnese in the south. Simeon proclaimed himself "Tsar (Caesar) of the Bulgarians and the Greeks," a title which was recognised by the Pope, but not of course by the Byzantine Emperor.