This is an interesting incident that Mormon relates with minimum detail. Mormon’s interest is in the retaking of Nephihah, but he does give us the information that while Moroni’s newly enlarged army is marching to Nephihah, that they meet, and defeat, a Lamanite army that they find in the field.
Why does Mormon add this incident? Possibly, it is because of the way that Moroni dealt with the captured Lamanites. Mormon has often pointed out that Moroni was not bloodthirsty. Here again we see him extracting a covenant from the captives rather than killing them. When the captives accepted the covenant, they were sent to be among the Ammonites, who were a people who did not even have weapons that the Lamanites might take and, therefore, attempt to fight against the Nephites again.
Mormon admires Moroni as a military genius, but also as a man of God. Mormon certainly understood the need for both traits, as Mormon also had to walk that line between living the gospel and defending his people through might of arms.