Complex sentence: 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.
Split into simpler sentences: While Louis's armies were fighting in Poland and against the Tatars, Louis marched to Croatia in June 1345. His troops besieged Knin, the former seat of the late Ivan Nelipac, who had successfully resisted Louis's father, forcing his widow and son to surrender.