Presian's reign coincides with the extension of Bulgarian control over the Slavic tribes in and around Macedonia. It is known that the Serbs and Bulgars lived in peace until Presian invaded Serbian territory in 839, during the last years of Theophilos. Vlastimir, the Knez of Serbia, united several Serbian tribes, and Emperor Theophilos (r. 829–842) granted the Serbs independence; thus, the Serbs acknowledged the nominal overlordship of the Emperor. The annexation of western Macedonia by the Bulgars changed the political situation. Malamir or Presian may have seen a threat in the Serb consolidation, and opted to subjugate them in the midst of the conquest of Slav lands. Another reason might have been that the Byzantines wanted to divert attention in order to cope with the Slav Uprising in the Peloponnese, and so sent the Serbs to instigate the war. Presian invaded Serbian territory in 839. The invasion led to a 3-year war, in which Vlastimir was victorious; Presian was driven out of Serbian territory by Vlastimir's army. In addition to making no territorial gains, Presian suffered heavy losses due to the Serbian tactical advantage in the hills. The war ended with the death of Theophilos in 842, which released Vlastimir from his obligations to the Byzantine Empire.