According to John Tvedtnes, anyone can make a promise to return to a particular subject and give further details, but it if Joseph Smith would have been making up the Book of Mormon narrative as he went along in dictation (which he dictated only once through to Oliver Cowdery) it would have been difficult for Joseph Smith to fulfill all such promises. As an abridger of the Nephite records, Mormon makes several important promises which are later fulfilled. One example is found in Alma 35:13. Here Mormon promised to give an account of the Nephite-Lamanite war which began in the eighteenth year, but since he proposed to first copy Alma's admonitions to his sons (Alma 36-42), he deferred the story of the war until Alma 43:3, where he introduced the topic by the words, "And now I return to an account of the wars." [John A. Tvedtnes, "Mormon As an Abridger of Ancient Records," in The Most Correct Book, p. 9]
Geographical [Theory Map]: Alma 35:13 The People of Ammon in Jershon Come over into Melek (17th Year)