Relations with the Byzantine Empire worsened after the death of Peter'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. The Byzantines encouraged the Rus' ruler Sviatoslav to attack Bulgaria, leading to the defeat of the Bulgarian forces and the occupation of the northern and north-eastern part of the country by the Rus' for the following two years.