Textual: Mormon is providing a context for Moroni’s actions that includes divine sanction. This is a continuation of the idea that he feels a need to justify their actions. He has done so from his perspective, but he needs to justify these actions from God’s perspective as well, particularly since Alma’s answer from the Lord indicates that the Lord is actively participating in these actions that might, in other circumstances, have been morally questionable.
Unfortunately, we do not have the source for this citation. It is not in our current Book of Mormon, no is it in other revealed scriptures. One would suspect that this is information communicated to Lehi, as at that early time the question of how they should relate to potential enemies would have been an extremely important question.
The principle that the Lord gives is that they may defend themselves with violence, but the acceptance of this type of violence does not extend to offensive actions. The Nephites may defend themselves vigorously, but not for the sake of the violence. They are not to immediately assume a violent response, but rather to respond lethally after the second offense. What we do not see in this case is the first offense for which this would be the second. Clearly this admonition is not seen as applying to every circumstance, but the “second” offense could be learned from the prior actions of the first offense. Having been attacked by Lamanites previously, their intent was known, and thus Moroni was justified in this attack on the Lamanite armies because the previous attacks had given notice of their intent. Even though Moroni begins the engagement, Mormon still sees this as a “second” offense, based on prior Lamanite actions.