The casus belli appears to have been an incident mentioned sub anno 6483 (975), in which Oleg of Dereva killed the Kievan boyar Lyut' (Old East Slavic: Лютъ) for tresspassing on his hunting grounds.

[3] Upon receiving news of Oleg's demise, Yaropolk pushed on towards Novgorod to defeat his other brother Volodimer, who fled to the Varangians in Scandinavia.

"[5][6] The PVL then narrates that after landing ashore, Volodimer sought to arrange a marriage with Rogned', daughter of prince Rogvolod of Polotsk, in an apparent move to gain more allies against Yaropolk.

[1] Upon her allegedly haughty refusal, Volodimer took Polotsk by force, killed Rogvolod and his two sons, abducting and forcibly marrying Rogned' anyway.

[9] Most modern scholars agree that this later story was invented for political purposes, deriving from a later Novgorodian tradition that tried to assert the superiority of Yaroslav's descendants over Rogvolod's.