Sviatoslav I of Kiev
With financial encouragement from the Byzantine emperor, Nikephoros II Phocas, Sviatoslav I of Kiev agreed to assist the Byzantines in their war with the Bulgarians. Sviatoslav defeated the Bulgarians (led by Boris II) and proceeded to occupy the whole of northern Bulgaria. He occupied Dobruja in 968 and moved the capital of Kievan Rus' to Pereyaslavets, in the north of the region. Sviatoslav refused to turn his Balkan conquests over to the Byzantines, and the parties fell out as a result. So the Byzantines under John I Tzimisces reconquered Dobruja in 971 and included it in the theme 'Mesopotamia of the West' (Μεσοποταμια της Δυσεον).