Sviatoslav's invasion of BulgariaPlovdiv, Bulgaria
Sviatoslav's invasion of Bulgaria refers to a conflict beginning in 967/968 and ending in 971, carried out in the eastern Balkans, and involving the Kievan Rus', Bulgaria, and the Byzantine Empire. 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. The allies then turned against each other, and the ensuing military confrontation ended with a Byzantine victory. The Rus' withdrew and eastern Bulgaria was incorporated into the Byzantine Empire.
In 927, a peace treaty had been signed between Bulgaria and Byzantium, ending many years of warfare and establishing forty years of peace. Both states prospered during this interlude, but the balance of power gradually shifted in favour of the Byzantines, who made great territorial gains against the Abbasid Caliphate in the East and formed a web of alliances surrounding Bulgaria. By 965/966, the warlike new Byzantine emperor Nikephoros II Phokas refused to renew the annual tribute that was part of the peace agreement and declared war on Bulgaria. Preoccupied with his campaigns in the East, Nikephoros resolved to fight the war by proxy and invited the Rus' ruler Sviatoslav to invade Bulgaria.
Sviatoslav's subsequent campaign greatly exceeded the expectations of the Byzantines, who had regarded him only as a means to exert diplomatic pressure on the Bulgarians. The Rus' prince conquered the core regions of the Bulgarian state in the northeastern Balkans in 967–969, seized the Bulgarian tsar Boris II, and effectively ruled the country through him.