Moroni didn’t need a long memory to understand what a significant number of defectors to the Lamanites might mean. He had just fought a significant battle against Zoramite fortified Lamanite armies, armies that were incited to battle because of the Zoramites. He could easily see the same situation occurring again, and so he decided not to allow this body of Amalickiahites to easily pass to the Lamanites.
Geographic: While specifics of geography are not given, Moroni’s assumption that he might “cut off” the route of the Amalickiahites suggests that they were north of Zarahemla, or in the downriver locations suggested above. This would put Moroni and Zarahemla further south, in the path that the Amalickiahites would have to travel to reach the land of Nephi, which was yet further south. The overall picture of the downriver locations being those with the greatest tendencies to dissent would appear to be confirmed here.