During the late 9th and early 10th centuries, Tsar Simeon I achieves a string of victories over the Byzantines, and expands the Bulgarian Empire to its apogee.

After the annihilation of the Byzantine army in the battle of Anchialus in 917, the Bulgarians lay siege to Constantinople in 923 and 924.

After the adoption of Christianity in 864, Bulgaria becomes the cultural center of Slavic Europe.

Its leading cultural position is further consolidated with the invention of the Cyrillic script in its capital Preslav, and literature produced in the Old Bulgarian language soon begins spreading North.

Old Bulgarian becomes the lingua franca of Eastern Europe, where it comes to be known as Old Church Slavonic.

In 927, the fully independent Bulgarian Patriarchate is officially recognized.Between the 7th and 10th centuries, the local population, the Bulgars and the other tribes in the empire, which are outnumbered by the Slavs, gradually becomes absorbed by them, adopting a South Slav language.

From the late 10th century, the names Bulgarians and Bulgarian become prevalent and become permanent designations for the local population, both in the literature and in the spoken language.

The development of Old Church Slavonic literacy has the effect of preventing the assimilation of the South Slavs into neighboring cultures, while stimulating the formation of a distinct Bulgarian identity.