Textual: This verse should be understood in the context of the sentence that immediately precedes it: “… thus commenced a war betwixt the Lamanites and the Nephites, in the eighteenth year of the reign of the judges; and an account shall be given of their wars hereafter.” Mormon has a complex story to tell, and he needs to find ways to build simultaneous threads. As usual, his primary interest is in the gospel, but he is also building his story to an important phase where war will become the major theme of his narrative.
At this point, he is not ready to move to the war narratives, nevertheless this incident with the Zoramites is a key trigger. He begins this thread, and then tells us to hold it in memory, as it will not be continued. It is only after beginning and holding the war thread that he returns to his gospel story. That story will next include the directives of Alma to his sons, after which the war narrative will begin is remarkable detail.