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.Gjuzelev, p. 130
