Mormon wrote this chapter header. Our current scripture formatting places it before the chapter designation, perhaps to separate it from the more modern synopses that appear in italics, but which were not part of the original text.
Mormon uses chapter headers to indicate that he has changed his source from the large plates of Nephi to some other record. This header, itself, does not declare what the source is, but we find it in verse 2. It is “according to his own record.”
When Alma abdicated as chief judge, he kept the portion of the large plates upon which he was recording official history. In addition, he kept a personal record of his missionary journey. Mormon uses this personal record heavily in the first part of the book of Alma, as it contains powerful sermons. When he returns to the large plate record that Alma kept, it will be to descriptions of wars and contentions. As a simple division, the great sermons in the book of Alma come from Alma’s personal record and the war chapters come from the large plates of Nephi, as kept by Alma as the official Nephite recordkeeper.