This war is different from any that had previously been fought. It was no longer the aim of the enemy to conquer; it was to destroy. This type of warfare is expensive, and there must be sufficient reason to justify the costs.
In a Mesoamerican context, the Nephites had controlled access through the narrow neck of land, thus potentially controlling trade that would go between the lands northward and the lands southward. The major populations at the time of this war of destruction were Teotihuacan in the north and the various Maya nations in the south. This was a time where trade was increasing, something that we begin to see in Mormon’s record: “And it came to pass that the robbers of Gadianton did spread over all the face of the land; and there were none that were righteous save it were the disciples of Jesus. And gold and silver did they lay up in store in abundance, and did traffic in all manner of traffic” (4 Nephi 1:46).
The increased trade suggests the need for stable trade routes from north to south, and the Nephites were apparently either hindering that trade, or in a position to do so. That would be unacceptable to the powers in the north and the south. Hence, the Gadiantons (north) and the Lamanites (south) were combined in this final war. It was economically beneficial to both to eliminate the threat that the Nephites posed, or could pose.