Mormon skims over this conflict. We do not know why the Lamanites were in the field. Perhaps they were a contingent being sent to accept the surrender of Zarahemla by the kingmen, an event precluded by Moroni’s victory over the kingmen. In Mormon’s brief account we simply learn that Moroni is victorious, and that he does not take prisoners, but rather extracts an oath. Of course we have seen Moroni accept the oath of a defeated army in a prior engagement (Alma 45:20). Moroni expected those who took the oath to obey it, and this prevented the necessity of having to guard these defeated men as prisoners.
It is significant that Moroni takes from the defeated army the provisions and the weapons. The confiscation of the weapons not only denied them to the Lamanites who had taken the oath, but also strengthened the military capacity of Moroni’s forces, many of whom were likely men of the fields who had brought their hunting armaments with them, and may not have had offensive weapons such as would have been carried by a formal war division, such as the Lamanites were. The provisions, of course, were always important to an army, but particularly so to one on the move and entering an area controlled by a hostile force.