Helaman and his brethren step into this situation. Moroni is the military man, but Helaman and his brothers are the religious leaders. These are the men who understand the things of the soul, not necessarily the things of war. It would not be surprising had Moroni been willing to accept the offer of the people of Ammon. After all, these were people who were probably trained in military arts. Nevertheless, it is Helaman and his brethren who intervene, and they intervene for religious reasons.
The people of Ammon had made and oath to God, and such oaths should be taken seriously. This one was an essential part of the conversion process for this people, and one that had already cost them personal loss of life. Helaman understands that for this people the oath was a tremendous, but essential burden. Helaman does not allow them to raise arms because he “feared lest by so doing they should lose their souls.” That is a tremendous consequence to breaking their oath, and probably one that they understood, and were yet willing to accept. They had sacrificed for their oath, and now were willing to sacrifice all, even more than their lives, for their brethren.