Having mentioned Alma’s children, Mormon specifically mentions Helaman along with the other sons of Alma. Mormon notes that while Moroni was doing his best militarily, these righteous men also assisted in the cause by preaching the gospel. Mormon attributes the peace in the land to the righteous preaching of these brothers. We remember that Alma had apparently taught them that the best way to calm contentions was through the preaching of the word (Alma 31:5).
Historical: Mormon notes that the effectiveness of the preaching brought four years of peace. These four years refer to four years from this time, and explicitly refer to internal peace rather than external peace. While we see conflict with the Lamanites, we do not get internal difficulties again for four years (Alma 50:25).