While Louis's armies were fighting in Poland and against the Tatars, Louis marched to Croatia in June 1345 and besieged Knin, the former seat of the late Ivan Nelipac, who had successfully resisted Louis's father, forcing his widow and son to surrender. The counts of Corbavia and other Croatian noblemen also yielded to him during his stay in Croatia. The citizens of Zadar rebelled against the Republic of Venice and accepted his suzerainty.

While his envoys negotiated in Italy, Louis marched to Dalmatia to relieve Zadar, but the Venetians bribed his commanders. When the citizens broke out and attacked the besiegers on 1 July, the royal army failed to intervene, and the Venetians overcame the defenders outside the walls of the town. Louis withdrew but refused to renounce Dalmatia, although the Venetians offered to pay 320,000 golden florins as compensation. Lacking military support from Louis, however, Zadar surrendered to the Venetians on 21 December 1346.