Following the death of Grand Prince Igor in 945, his wife Olga ruled as regent in Kiev until their son Sviatoslav reached maturity (ca. 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.