Béla's was on good terms with the Holy Roman Empire, jeopardizing the interests of Boleslaw III of Poland who had been warring with the empire. The Polish monarch decided to support a pretender to the Hungarian crown named Boris. After Boris arrived in Poland, a number of Hungarian noblemen joined him.

Accompanied by Polish and Rus' reinforcements, Boris broke into Hungary in mid-1132. Béla entered into an alliance with Leopold III, Margrave of Austria. Before launching a counter-attack against Boris, Béla convoked a council on the river Sajó. The Illuminated Chronicle relates that the King asked "the eminent men of Hungary" who were present if they knew whether Boris "was a bastard or the son of King Coloman". The King's partisans attacked and murdered all those who proved to be "disloyal and divided in their minds" during the meeting.

Béla tried to persuade the Polish monarch to stop supporting the pretender. However, Boleslaw remained loyal to Boris. In the decisive battle, which was fought on the river Sajó on 22 July 1132, the Hungarian and Austrian troops defeated Boris and his allies.