Presian's Campaign

The first war between Bulgarians and Serbs broke out between 839 and 842. Before this conflict, the two peoples had lived peacefully, according to Byzantine sources. The problem arose from Byzantium's policy of diverting Bulgarian expansion away from its southwestern provinces. When the Bulgarians captured western Macedonia, the Serbian tribes felt threatened by the powerful First Bulgarian Empire. Vlastimir, prince of the Serbian tribes, managed to unite several tribes, with support from the Byzantine emperor Theophilos to guarantee Serbian independence. This created a threat to the Bulgarians. Khan Presian decided to end Byzantine influence over the Serbs and attacked them in 839. 

The war lasted three years, with neither side able to achieve a decisive victory. In 842, the border between the states was established at the mouth of the Drin River.

After the war with Bulgaria ended, Serbia launched unsuccessful campaigns westwards to subjugate the Croats. Meanwhile, the Byzantines managed to deal with Slavic rebellions in the Peloponnese by diverting Bulgarian attention away from the Balkans.

The war ended with Theophilos's death in 842, releasing Vlastimir from his obligations to the emperor and allowing the Bulgarians to attack the Byzantine Empire and annex the region around Ohrid, Bitola and Devol in 842–843.