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 the invasion in 839 (the last years of Theophilos).

Vlastimir, the Knez of Serbia, had united several Serbian tribes, and Emperor Theophilos (r. 829–842) grants the Serbs independence, thus the Serbs acknowledge nominal overlordship of the Emperor.

The annexation of western Macedonia by the Bulgars changes the political situation: Presian may have seen a threat in the Serb consolidation, and opted to subjugate them in midst of the conquest of Slav lands.

Another cause might have been that the Empire wanted to divert the attention so that they could cope with the Slav Uprising in the Peloponnese, meaning they would have sent the Serbs to instigate the war.

Presian invades Serbian territory between 839–842 .

The invasion leads to a three-year war, in which Vlastimir is victorious; Presian makes no territorial gains, is heavily defeated and loses many of his men as the Serbs have a tactical advantage in the hills; he is driven out by the army of Vlastimir.

The war ends with the death of Theophilos in 842, which releases Vlastimir from his obligations to the Empire.