Nephi is pretty consistent in using the phrase “seed of my brethren” to indicate Lamanites—not Lamanites as a lineage, but as a designation of those who are not Nephites. Thus, he sees the Lamanites defeat the Nephites.
The history from that point will cover the time after then end of the Nephites and prior to the coming of the Europeans. Part of that history is a continuation of the Lamanite association with wars. Even without Nephites to battle, the Lamanites will continue to have wars.
It is not clear just how Nephi understood that after this point the Lamanites would dwindle in unbelief. Perhaps Nephi had not clearly seen the degree to which the later Nephite writers would chronicle Lamanite unbelief. Perhaps, it was their comparative righteousness at the time of the Nephite demise that allowed them to defeat the Nephites.
Whatever that meant, it is only after the demise of the Nephites that Nephi sees them become “a dark, and loathsome, and a filthy people, full of idleness and all manner of abominations.” Whatever Nephi meant with those words, he doesn’t apply them until after there are no Nephites. Other Nephite writers, however, will use them for the Lamanites who are their contemporaries.
There is no chapter break at this point in the 1830 edition.