Casimir III of Poland urged Louis to intervene in his war with the Lithuanians who had occupied Brest, Volodymyr-Volynskyi, and other important towns in Halych and Lodomeria in the previous years. The two monarchs agreed that Halych and Lodomeria would be integrated into the Kingdom of Hungary after Casimir's death. Casimir also authorized Louis to redeem the two realms for 100,000 florins if Casimir fathered a son. Louis led his army to Cracow in June 1351. Because Casimir fell ill, Louis became the sole commander of the united Polish and Hungarian army. He invaded the lands of the Lithuanian prince, Kęstutis, in July. Kęstutis seemingly accepted Louis's suzerainty on 15 August and agreed to be baptised, along with his brothers, in Buda. However, Kęstutis did nothing to fulfill his promises after Polish and Hungarian troops were withdrawn. In an attempt to capture Kęstutis, Louis returned, but he could not defeat the Lithuanians, who even killed one of his allies, Boleslaus III of Płock, in battle. Louis returned to Buda before 13 September. A papal legate visited Louis to persuade him to wage war against Stefan Dušan, Emperor of the Serbs, who had forced his Roman Catholic subjects to be re-baptised and join the Serbian Orthodox Church.