Nephi was given the power to call a famine in Helaman 10:6. In verse 5 he invokes that power. Verse 5 also tells us that the famine worked such that “the work of destruction did cease by the sword.” Why did the famine have this effect?
In the ancient world all food was produced locally. When an army marched, they were called up out of the fields, and there were fewer people to till. There were also more mouths to feed that were not growing food. The longer an army was actively in the field, the longer the rest of the people were required to produce more than their needs, or do with less, in order to feed the army. When a famine came, there wasn’t extra food, and all were needed to till the ground to simply support themselves.
The famine caused the warring to end because there wasn’t food necessary to sustain war. Mormon tells us that not only was there not enough to sustain war, there was hardly enough at all. This was a severe famine. Many centuries later, famine played a role again in reducing the ability of a people to engage in war. One of the factors aiding the Spanish conquest of the Yucatan was a drought-enduced famine that killed large numbers of people (Diego de Landa. Relacion de las cosas de Yucatan. Editorial Porrua, Mexico, 1973, p. 25).