With the southeast corner of the land now without a strong defense, the combined armies of the Zoramites and the Lamanites take advantage of this. This was not the typical entry point for Lamanite invasions, which normally came through what must have been a pass near the city of Manti. However, the defection of the Zoramites opened a new possibility, and this war will end up being fought on two fronts.
Mormon certainly speaks of the Lamanites as the enemy of the Nephites, but he also takes pains to show his readers that even more dangerous than hereditary Lamanites were those who were apostate Nephites who had become Lamanites. That is the case here. At least one people designated as Amalekites had combined with the Lamanites and the former priests of Noah to create a city named Jerusalem in Lamanite territory (Alma 21:2). That they were of the order of Nehor (Alma 21:4) suggests that they were once Nephites. They were among those who most strongly resisted the preaching of the sons of Mosiah. Mormon makes certain that his readers understand that Zerahemnah, the leader of these armies, was both an Amalekite and one with a particular “hatred towards the Nephites.”
From a historical standpoint, it is interesting that Mormon tells us that Zerahemnah desired that “he might gain power over the Nephites by bringing them into bondage.” The Book of Mormon uses the term bondage to indicate a tribute relationship to a more distant overlord. Thus, this isn’t a conquest for new territory, but is an attempt to create a client state that would essentially pay taxes to the overlord. In this case, the overlord would be the Amalekites. Presumably, the Zoramites would also receive some of those taxes in recompense for their part in the war.