Despite initial reluctance from some Polish lords, Casimir led his forces into battle against opposing Rus princes.

The engagement culminated in a decisive victory for Casimir,[citation needed] resulting in the surrender of Ruthenians and the restoration of Mstislav[which?]

of the city of Brest and the land located on the Bug River immediately motivated Prince Casimir to take up arms.

[2]: 156 [3]: 209 [7][4][6] After sentencing the perpetrators of the breakaway to be beheaded, he build a fortress there towering over the city and, having placed a strong garrison in it to keep the people in obedience.

[2]: 157 [3]: 211 Then the Poles no longer secretly but openly reproach their prince Casimir, and hurl insults at the palatine and governor of Cracow Mikołaj, as the proper perpetrator of this war.

The Ruthenians were aroused to fight, by the clearly small handful of them and the great multitude of their own troops, as well as by fortune-tellers asking about the outcome of the war, proclaiming that everything would succeed for them and that the Poles would meet a sad end.

[2]: 158 [7][4] The town and city of Halych also immediately surrendered to the victorious Casimir, who handed it over to his affinity Prince Mstislav.

[1]: 153 [3]: 214 [6][8] Having induced him beforehand to swear to him that he would never, either in misfortune or good luck, abandon him and the Polish nation, he restored the royal throne in Halych to him and, having put things in order in Rus, returned to Poland.