The destruction of the Lamanites will come with “wars and pestilences… famines and bloodshed.” This will be the end of the Nephite world. The cause of this destruction will be that “they shall dwindle in unbelief and fall into the works of darkness…” Both Alma and Helaman would immediately understand this as the true cause for the destruction of their people. Once they lost their commitment to the God of the Nephite religion, the identity of Nephite people would be lost. Remember that we have seen that the Nephites feared that mixing with Lamanite people and ideas would lead to the destruction of the Nephites:
Alma 3:8
8 And this was done that their seed might be distinguished from the seed of their brethren, that thereby the Lord God might preserve his people, that they might not mix and believe in incorrect traditions which would prove their destruction.
The final destruction of the Nephites will be accompanied by bloodshed, but the cause will be spiritual, not military. It is the “incorrect traditions” that the Nephites will eventually adopt that will “prove their destruction.”