In Alma 25:1 it says, "behold, now it came to pass that those Lamanites were more angry because they had slain their brethren; therefore they swore vengeance upon the Nephites." What Nephites were the Lamanites so upset with? Was it all Nephites wherever they might live? If the Anti-Nephi-Lehi's did not fight back, and if the main part of the slaughter was done by Amalekites and Amulonites (Alma 24:28), then why were the Lamanites so upset with Nephites? What was their motivation? And if they were upset with the Nephites in general, why would they travel all the way to the city of Ammonihah, which was three days journey north of the land of Melek (Alma 8:6), before they started destroying Nephites? [Alan C. Miner, Personal Notes]
“They Swore Vengeance Upon the Nephites”
According to Brant Gardner, with the Anti-Nephi-Lehi movement crushed and their royal protector (Anti-Nephi-Lehi, the former Lamanite king) out of the way, the Lamanites were ready to seat their own king. In the canons of known Maya kingship rituals, the seating of the king was intimately linked with the capture and sacrifice of prisoners of war. The translation of the glyphs makes it clear that captives were an important part of the seating rituals. With the passive resistance of the Anti-Nephi-Lehies, there would have been no war captives to use in the ascension of the new king. Under such circumstances, it would make sense that the army that was still intact, and lacking the real combat they had expected, would be willing to engage in another battle to supply the requisite ritual prisoners for the seating of the new king. Indeed, when we read the story of this invasion in Alma 16 we read that captives were taken, and were being taken back to Lamanite territory.
It is noteworthy that the attack on Ammonihah lacks any attempt to establish tribute or a Lamanite outpost. Indeed, this would have been virtually impossible, given the location of Ammonihah so deep in Nephite territory. This war had only one real purpose, and that was the acquisition of the captives that were being led back to the land of Nephi. The need for these captives in the situation of the installation of a new Maya king provides a direct parallel to the situation described in the Book of Mormon. [Brant Gardner, Book of Mormon Commentary, [http://www.highfiber.com/~nahualli/LDStopics/Alma/Alma25.htm], pp. 1-2]