The Nephite general Moronihah had abandoned any further attempt to reclaim Nephite lands lost to the Lamanites. There was no military solution.
In chapter 5, Mormon notes that "they who chose evil were more numerous than they who chose good, therefore they were ripening for destruction." (v. 2) So, Nephi did the following:
4 And it came to pass that Nephi had become weary because of their iniquity; and he yielded up the judgment-seat, and took it upon him to preach the word of God all the remainder of his days, and his brother Lehi also, all the remainder of his days;
But here is the miracle. Nephi and Lehi went to Zarahemla, the great Nephite capital which was now possessed by the Lamanites, and they baptized more than 8,000 Lamanites. They also baptized Nephite dissenters who had been so much trouble and had inspired war and conflict. Those converts returned to the land Bountiful and ministered to the Nephites.
Then Nephi and Lehi went to the land of Nephi where they were imprisoned and sentenced to die. A great miracle occurred that led to the conversion of 300 who then taught others until the majority of the Lamanites were converted. In a marvelous manifestation of the Holy Spirit, a man named Aminadab who had been a Nephite and who had fallen away, took it upon himself to teach the others what this great manifestation meant. There are many Aminadab's in the church of God. They have wandered away, sometimes with bitter hearts. But they can be returned to the fold. We need to look for the Aminadabs among us.
What military might could not do was accomplished through the power of the word of God taught by humble servants of God.
50 And it came to pass that they did go forth, and did minister unto the people, declaring throughout all the regions round about all the things which they had heard and seen, insomuch that the more part of the Lamanites were convinced of them, because of the greatness of the evidences which they had received.
51 And as many as were convinced did lay down their weapons of war, and also their hatred and the tradition of their fathers.
52 And it came to pass that they did yield up unto the Nephites the lands of their possession.