The Lamanite invasion violated a solemn treaty. The question between the kings therefore focuses on that violation. The treaty oath should only have been breached for cause. Upon finding out that there was indeed a breach of the treaty, Limhi declares that he will search out the perpetrators among his people.
In short, rather than declaring himself the winner and rejoicing over his conquered opponent, Limhi focuses, not on the battle, but on a broken oath. When Limhi learns that his own people might have been the oath-breakers, the entire situation changes. Rather than triumphant victors, the Limhites are now tainted with shame. Limhi’s honor requires the resolution of this situation. The issue is no longer warfare but integrity.