The Lamanites now have an army at their rear, though across a river and not an immediate threat. They also had an army in front of them, and they were not winning the battle. Once again, they flee. This time they proceed down the river valley in the land of Manti, heading toward Zarahemla.
The original description of Moroni’s action divided the army in two. Where did this third army come from? The best answer is that the army that Moroni brought with him was the one divided, and the one that was in the land on the south was the army raised from the men of that land. These were men who were directly defending their homes, and therefore they were highly motivated, if not particularly trained.