Bulgaria under rule of Boris I
When Khan Boris began his reign in 852, the international situation in Southeast Europe was characterized by a race for influence in the region, both cultural and political. The conflict with the Byzantine Empire for domination over the Slavic tribes in modern-day Macedonia and Thrace was still far from being resolved. In the middle Danube region, Bulgaria's interests crossed with those of the emerging kingdom of the East Franks and the principality of Great Moravia. It was about that period when Croatia emerged on the international scene, carrying its own ambitions and demands for territories in the region.
On a larger scale, the tensions between Constantinople and Rome were tightening. Both centres were competing to lead the Christianization that would integrate the Slavs in South and Central Europe. The Bulgarian Khanate and the Kingdom of the East Franks had established diplomatic relations as soon as the 20s and 30s of the 9th century. In 852, at the beginning of the reign of Khan Boris, a Bulgarian embassy was sent to Mainz to tell Louis II of the change of power in Pliska, the Bulgarian capital. Most probably the embassy also worked to renew the Bulgarian-German alliance.
Some time later, Khan Boris concluded an alliance with Rastislav of Moravia (846–870) instigated by the King of the West Franks, Charles the Bald (840–877). The German Kingdom responded by attacking and defeating Bulgaria, forcing Khan Boris to later re-establish an alliance with the German king directed against Great Moravia, a Byzantine ally. The situation held great risk for the weakened Bulgarian state. War broke out with the Byzantine Empire between 855 and 856. The Byzantines wanted to regain control over some fortresses on the Diagonal Road (Via Diagonalis or Via Militaris) that went from Constantinople, through Philippopolis (Plovdiv), to Naissus (Niš) and Singidunum (Belgrade). The Byzantine Empire was victorious and reconquered a number of cities, with Philippopolis being among them.
In 861 Khan Boris concluded an alliance with East Frankish King Louis the German, all while informing him that he would like to accept Christianity according to western rite. This renewed alliance threatened Great Moravia, which sought help from Byzantium (862–863). This was at the same time when a Byzantine mission to Great Moravia was taking place. Cyril and his brother Methodius intended to draw Great Moravia closer to Constantinople and strengthen the Byzantine influence there.
Khan Boris was more interested in the first Slavonic alphabet Cyril and Methodius had created. Bulgaria wanted to implement the Slavonic alphabet as well as a means to stop the cultural influence of the Byzantine Empire.
In the last months of 863 the Byzantines attacked Bulgaria again, probably after having been informed by their Moravian allies that Boris told the German king he was willing to accept Christianity and Byzantium had to forestall him from taking up Christianity from Rome. A Rome-dependent Bulgaria in the hinterland of Constantinople was a threat to the Byzantine Empire's immediate interests.