Boris inherits the Bulgarian throne from his father, Khan Presijan.

Bulgaria's territorial, military, and political potential have made it one of the largest states in Europe.

Bulgaria's approximate frontiers are the Dnieper River in the northeast, the Carpathian Mountains in the north, the Tisa (Tisza) River in the northwest, the Adriatic Sea in the west, and the Tomorr (Tomor), Belasica, Pirin, Rhodope, and Strandzha mountains in the south.

Many Slavic tribes had settled within the boundaries of the state, together with the proto-Bulgarians, a tribe of Turkic origin that had settled in the Balkan Peninsula at the end of the seventh century.

In view of the religious, ethnic, and language difficulties between the Slavs and the Bulgars, the introduction of a common and compulsory religion for all subjects is one of the principal preconditions for the formation of a united Bulgaria.

Pagan Bulgaria needs to join the “family of Christian states,” but the existence of two competing centers of Christianity—Rome and Constantinople—make it difficult for Boris to make his choice.

In 852, he sends emissaries to Eastern Francia to confirm the peace treaty of 845.

At the time of his accession he threatens Constantinople with an invasion, but his armies do not attack, and he receives a small area in Strandzha to the southeast.

The peace treaty is not signed, however, although both states exchange temporary delegations.