Relations with the Byzantine Empire worsened after the death of Petar's wife in the mid-960s.
Victorious over the Arabs, Emperor Nikephoros II Phokas refused to pay the annual tribute to Bulgaria in 966, complaining of the Bulgarian alliance with the Magyars, and he undertook a show of force at the Bulgarian border.
Dissuaded from a direct attack against Bulgaria, Nikephoros II dispatched a messenger to the Rus prince Sviatoslav Igorevich to arrange a Rus attack against Bulgaria from the north.

Sviatoslav readily launched a campaign with a vast force of 60,000 troops, routed the Bulgarians on the Danube, and defeated them in a battle near Silistra, seizing some 80 Bulgarian fortresses in 968.
Stunned by the success of his ally and suspicious of his actual intentions, Emperor Nikephoros II hastened to make peace with Bulgaria and arranged the marriage of his wards, the underage emperors Basil II and Constantine VIII, to two Bulgarian princesses.
Two of Petar's sons were sent to Constantinople as both negotiators and honorary hostages.
In the meantime Petar managed to secure the retreat of the Rus forces by inciting Bulgaria's traditional allies, the Pechenegs, to attack Kiev itself.

In spite of this temporary success and the reconciliation with Byzantium, Bulgaria faced a new invasion by Sviatoslav in 969.
The Bulgarians were defeated again, and Petar suffered a stroke, which led him to abdicate and become a monk.
He died on 30 January 970.
Relations with the Byzantine Empire worsened after the death of Petar's wife in the mid-960s. Victorious over the Arabs, Emperor Nikephoros II Phokas refused to pay the annual tribute to Bulgaria in 966, complaining of the Bulgarian alliance with the Magyars, and he undertook a show of force at the Bulgarian border. Dissuaded from a direct attack against Bulgaria, Nikephoros II dispatched a messenger to the Rus prince Sviatoslav Igorevich to arrange a Rus attack against Bulgaria from the north.
Sviatoslav readily launched a campaign with a vast force of 60,000 troops, routed the Bulgarians on the Danube, and defeated them in a battle near Silistra, seizing some 80 Bulgarian fortresses in 968. Stunned by the success of his ally and suspicious of his actual intentions, Emperor Nikephoros II hastened to make peace with Bulgaria and arranged the marriage of his wards, the underage emperors Basil II and Constantine VIII, to two Bulgarian princesses. Two of Petar's sons were sent to Constantinople as both negotiators and honorary hostages. In the meantime Petar managed to secure the retreat of the Rus forces by inciting Bulgaria's traditional allies, the Pechenegs, to attack Kiev itself.
In spite of this temporary success and the reconciliation with Byzantium, Bulgaria faced a new invasion by Sviatoslav in 969. The Bulgarians were defeated again, and Petar suffered a stroke, which led him to abdicate and become a monk. He died on 30 January 970.
Peter I of Bulgaria