Although Mormon does not tell us, we can surmise that Moroni has been able to blunt Ammoron’s attack on the west. That advance must be bogged down and under sufficient control that Moroni can march to Bountiful to assist Teancum. He needs to do this because the Lamanites in that quarter of the land are in the Nephite cities, and thus have a much stronger base of operation for a long term conflict than does the army of Ammoron. The threat in that quarter is very great, and once the basic border is at least temporarily stabilized, Moroni must go to the area of greatest threat. The specific threat is the city of Mulek, which appears to house the largest of the Lamanite divisions.