Zeniff leads his people against the Lamanites. It is unknowable whether or not Zeniff was aware of who was responsible for the raid. Since all enemies were designated Lamanites, his retaliation could have been against anyone who might have been called an enemy. Ironically, this same mistake of blaming a whole people for what some of them did will happen again in the story of the priests of Noah and the daughters of the Lamanites.
In this case, the people of Zeniff are successful in their retaliation. They suffered significant loss, but even worse was that this appears to begin the rapid deterioration of the originally peaceable relationship between the people of Zeniff and the local Lamanites.
There is no break at this point in the original edition of the Book of Mormon. All of Zeniff’s personal writings formed a single chapter.