Redaction: The context Mormon provides for Moroni’s actions includes divine sanction, continuing to justify Moroni’s actions. He has done so from his own perspective, but he also does so from Yahweh’s perspective. This element is important. Alma’s revelation about the Lamanites’ battle plan indicates that Yahweh is actively participating in events that might, in other circumstances (as Mormon implies), have been morally questionable.
This quotation is not in any extant scripture. Presumably, it was a revelation to Lehi, since the question of how to relate to potential enemies would have been extremely important to him. Yahweh’s principle was that lethal force is justifiable in self-defense but that offensive actions are not acceptable. To be sure that the Nephites do not immediately make a violent response (let alone a preemptive strike), they should wait to counter until after the second offense.
Even though Moroni began the engagement with his ambush, Mormon still saw it as a “second” offense. Apparently, the massing of the Lamanite army at Antionum and the obvious but aborted intent to attack at Jershon constituted the first offense.