power. Igor returned to Kiev keen for revenge. He assembled a large force of warriors from among neighboring Slavs and Pecheneg allies, and sent for reinforcements of Varangians from "beyond the sea." In 944 the Rus' force advanced again on the Greeks, by land and sea, and a Byzantine force from Cherson responded. The Emperor sent gifts and offered tribute in lieu of war, and the Rus' accepted. Envoys were sent between the Rus', the Byzantines, and the Bulgarians in 945, and a peace treaty was completed. The agreement again focused on trade, but this time with terms less favorable to the Rus', including stringent regulations on the conduct of Rus' merchants in Cherson and Constantinople and specific punishments for violations of the law. The Byzantines may have been motivated to enter the treaty out of concern of a prolonged alliance of the Rus', Pechenegs, and Bulgarians against them, though the more favorable terms further suggest a shift in power. Sviatoslav Following the death of Igor in 945, his wife Olga ruled as regent in Kiev until their son Sviatoslav reached maturity (c. 963). His decade-long reign over Rus' was marked by rapid expansion through the conquest of the Khazars of the Pontic steppe and the invasion of the Balkans. By the end of his short life, Sviatoslav carved out for himself the largest state in Europe, eventually moving his capital from Kiev to Pereyaslavets on the Danube in 969. In contrast with his mother's conversion to Christianity, Sviatoslav, like his druzhina, remained a staunch pagan. Due to his abrupt death in an ambush in 972, Sviatoslav's conquests, for the most part, were not consolidated into a functioning empire, while his failure to establish a stable succession led to a fratricidal feud among his sons, which resulted in two of his three sons being killed. Reign of Vladimir and Christianisation It is not clearly documented when the title of the Grand Duke was first introduced, but the importance of the Kiev principality was recognized after the death of Sviatoslav I in 972 and the ensuing struggle between Vladimir/Volodymyr the Great and Yaropolk I. The region of Kiev dominated the state of Kievan Rus' for the next two centuries. The grand prince or grand duke ( or , , , ) of Kiev controlled the lands around the city, and his formally subordinate relatives ruled the other cities and paid him