Verse 1 originally followed verse 54 of Chapter 43 without a break. That verse described the end of the conflict, when Moroni commanded that his armies stop killing the Lamanites. It is at that time that “they did stop and withdrew a pace from them.” This allowed Moroni to approach Zerahemnah to give this speech.
We do not know how Moroni knew which Lamanite combatant was the leader, but it would not have been unusual in ancient warfare for that person to have some kind of visual marker that would identify him as a leader. Even modern military uniforms have visual indicators of rank. Thus, Moroni could approach the leader, who would be able to make a decision for the whole of his army.
The obvious evidence, to any ancient warrior, that God was on their side was that they were victorious. Thus, Moroni declares that “the Lord is with us; and ye behold that he has delivered you into our hands.” Moroni even uses their victory to declare that “ye see that this is the true faith of God.”