Because the Lamanites had been deceived by the Amalekites and the Amulonites whom they regarded as being Nephites, into believing that the people of Anti-Nephi-Lehi were enemies to the common good, and that the king, himself, was unfit to rule, they put all the blame for their misguided venture upon the heads of the Nephites no matter where they lived or who they were. They reasoned that all Nephites were responsible for their folly and their disaster. In their hatred of everything Nephite, which was like a seething cauldron of ill will, the Lamanites "swore vengeance" upon the Nephites, and in their spite and angry passions they temporarily forgot the Anti-Nephi-Lehites and went forth to slay the Nephites wherever they could be found. Their anger knew no bounds.
In furtherance of their wicked designs, in which the Lamanites imagined that their injuries were caused by the Nephites, they thought that the deceptions foisted upon them by the Amalekites and the Amulonites could be redressed only by inflicting adequate punishment upon every Nephite wherever and whenever one was found. Following this conclusion, the Lamanites gathered their armies together "and went over to the borders of the Land of Zarahemla." There they found a numerous people to whom Alma and Amulek ministered only a few months previously. The City of Ammonihah was the first populated area to feel the brunt of their attack. Before the Nephites in Zarahemla could muster an army for defense purpose, the Lamanites destroyed that great city and all the people therein. This was in fulfillment of a prophecy uttered by Alma: "But behold, I say unto you that if ye persist in your wickedness that your days shall not be prolonged in the land, for the Lamanites shall be sent upon you; and if ye repent not they shall come in a time when you know not, and ye shall be visited with utter destruction; and it shall be according to the fierce anger of the Lord." (Alma 9:18) The fulfillment of this prophecy is also noted by Alma in the Sacred Record: "And it came to pass in the eleventh year of the Reign of the Judges over the people of Nephi, on the fifth day of the second month, there having been much peace in the Land of Zarahemla, there having been no wars nor contentions for a certain number of years, even until the fifth day of the second month in the eleventh year, there was a cry of war heard throughout the land. For behold, the armies of the Lamanites had come in upon the wilderness side, into the borders of the land, even into the City of Ammonihah, and began to slay the people and destroy the city. And now it came to pass, before the Nephites could raise a sufficient army to drive them out of the land, they had destroyed the people who were in the City of Ammonihah, and also some around the borders of Noah, and taken others captive into the wilderness. (Ibid., 16:1-3) "And thus ended the eleventh year of the Judges, the Lamanites having been driven out of the land, and the people of Ammonihah were destroyed; yea, every living soul of the Ammonihahites was destroyed, and also their great city, which they said God could not destroy, because of its greatness. But behold, in one day it was left desolate; and the carcasses were mangled by dogs and wild beasts of the wilderness. Nevertheless, after many days their dead bodies were heaped up upon the face of the earth, and they were covered with a shallow covering. And now so great was the scent thereof that the people did not go in to possess the Land of Ammonihah for many years. And it was called Desolation of Nehors; for they were of the profession of Nehor, who were slain; and their lands remained desolate." (Ibid., 16:9-11. See also COMMENTARY ON THE BOOK OF MORMON, Volume III, Chapter 16)
The Sacred Record notes that after the destruction of the City of Ammonihah the Lamanites had a number of battles with the Nephites, in all of which the Lamanites were "driven and slain." No doubt the success of their surprise attack upon Ammonihah made the Lamanites bold, and at first, believing in easy victory, they sought out the Nephites and gave them battle. But, as we have seen, the Lamanites were eventually driven out of all the land.