Facing Bogomilism and rebellions by his brothers and also by Časlav Klonimirović early on in his reign, Peter secured more success later in life; he ensured the retreat of the invading Rus by inciting Bulgaria's allies, the Pechenegs, to attack Kiev itself.

Considered a good ruler during the Middle Ages, his name was adopted by later leaders trying to restore Bulgarian independence under Byzantine rule to emphasize legitimacy and continuity.

To prove himself a worthy successor to his father both at home and in the eyes of foreign governments, Peter began his reign with a military offensive into Byzantine Thrace in 927 which was the last campaign of the Byzantine–Bulgarian war of 913–927.

The Byzantine Emperor Romanos I Lakapenos eagerly accepted the proposal for peace and arranged for a diplomatic marriage between his granddaughter Maria and the Bulgarian monarch.

In October 927 Peter arrived near Constantinople to meet Romanos and signed the peace treaty, marrying Maria on 8 November in the church of the Zoödochos Pege.

The youngest brother, Benjamin (also called Bojan), was accused of being a werewolf and magician by the Italian Bishop Liutprand of Cremona, but he apparently posed no threat to Peter's authority.

In spite of the challenges he encountered soon after his accession and the critical situation at the very end of his life, Peter's Bulgaria appears to have been prosperous and increasingly well organized, with an administrative apparatus noted by foreign travelers and confirmed by the numerous finds of imperial seals.

Luxury and social tensions may have contributed to the spread of Bogomilism, which Peter duly undertook to combat, soliciting the advice of famous hermits and even his uncle-in-law, Patriarch Theophylact of Constantinople.

[3] While Peter's reign witnessed the spread of Bogomilism, its origins were more demographic (perhaps inspired by Paulicians settled earlier by Byzantine emperors in Thrace) than social, and the Bulgarian Orthodox Church canonized him as a saint.