Not many years previously, the Nephites had so given themselves over to new ideas that they rejected not only the gospel, but governance by righteous men who upheld the gospel. Now after great tribulation, the people have been purged, and true Nephites (in the sense that they believe in the gospel) are back at the head of the government. For Mormon, this is the picture of a return to righteousness, and therefore we have peace and prosperity. Therefore, when he mentions the righteous men as leaders, he accompanies that description with an attribution of the establishment of peace to those men.