We assume it was A.D. 376 when Mormon resumed leading the Nephite armies (v. 1). There are no dates identified between A.D. 375 and 379, but the events spoken of in the ending of chapter 4 could have taken place in one year, and the verses discussed under this heading are quite general.
Mormon refused to be their military leader in A.D. 362 (see Mormon 3:8–11). Therefore, if our assumption concerning the time is correct, he had not been leading the armies for thirteen or more years. He had undoubtedly been writing and abridging during this time. He does not give his reason for returning to the leadership. That he “did repent of the oath” (Mormon 5:1) means he reversed his decision, not that his previous decision was wrong or sinful. The people were just as wicked, and probably more wicked, but he came back knowing it was a lost cause (v. 2). However, his return apparently inspired the men to fight more fiercely for their cities, and they were successful (vv. 3–4). Regardless of the effort of the Nephite armies, the destruction by the Lamanites was progressing steadily (v. 5).