Seeing the defending army wearing protective gear changed the Amalekite army’s plans. They left the field and went south and west to enter Nephite lands through the more traditional entry point near Manti, an entry point that led directly to the heart of the land of Zarhemla.
Militarily, we see Moroni consulting with Alma to know where the Lamanites would go. Moroni likely knew that Zoram had done the very same thing, and Alma was able to tell him where to go to find the army fleeing after the destruction of Ammonihah (see Alma 16:5–6). Moroni gets an answer and is able to bring part of his army to bear at the entry point before the Amalekites could press their attack.
The geographic implication of this action is interesting. The Amalekites have the advantage of beginning their march first. Moroni has to wait to know where they were going. When he finds out, he has to get his army in place quickly. How does he get there faster? It appears that the circuit from the southeastern corner of the land of Nephi was longer and or more difficult than a journey through the Sidon River valley. Moroni’s path would have been shorter, and probably more easily traveled than the route taken by Zerahemnah.