Verse 2 explains that Mormon wrote a letter to the enemy king requesting the change to gather together in the land of Cumorah. Mormon must have believed that it would be a final battle, and the king likely agreed that it would be the final battle. Therefore, the king granted the request.
Even though Mormon had noted that the nature of warfare was becoming more destructive (particularly toward the women and children who were taken captive to be sacrificed), there were still rules of warfare. One of them allowed for battles that were scheduled. Thus, neither the request nor the granting of the request would have been out of place in a historical context, particularly a Mesoamerican context.
In the Nephite year three hundred and eighty-four the Nephites were gathered in Cumorah, and the end was officially beginning.