In 896, Klonimir returned from Bulgaria, backed by Tsar Boris, and invaded Serbia, taking the important city Destinikon, but was attacked and killed by Petar.

[7][8][9] After several failures to capture the throne by other Vlastimirovićs, including the one backed by the Bulgarians, Tsar Simeon I of Bulgaria recognized Petar as ruler.

[10] According to Constantine VII's De Administrando Imperio, Petar ruled under the suzerainty of Leo VI and was at peace with Bulgaria for twenty years.

Symeon had, in contrast to Tsar Boris, been schooled in Constantinople, embracing Byzantine ideology, and wanted to rule a joint Roman-Bulgarian Empire as Roman Emperor.

He expanded along the Neretva, annexing the Narentines, where he seems to have come into conflict with Michael Višević, the ruler of Zahumlje (with Travunija and most of Duklja), who was an important Bulgarian ally.

[13] Petar (since 897 theoretically a Bulgarian vassal, though not necessarily a willing one)[12] met with the strategos of Dyrrhachium, Leo Rhabduchus, in Neretva,[14] where he was offered money and greater independence[12] in exchange for leading an army (also containing Tourkoi, Magyars) against Symeon.

[13] After Achelous,[5] Symeon sent an army led by Pavle (the son of Bran), to take the Serbian throne, but failed, as Petar proved a good opponent.

[14] Symeon sent generals, Marmaim and Theodore Sigritzes,[16][17] persuading Petar (through an oath[14]) to come out and meet them,[17] then captured and took him to Bulgaria where he was put in prison, dying within a year.