Mormon is summarizing the first fifteen years and emphasizing the destruction of lives. The particular event that he refers to is this great battle in the fifteenth year that is the bloodiest yet with the Lamanites (verse 2). Nevertheless, he wants us to see all of the events from the first year of the reign of the judges to this point leading to this final point. It is true that there have been other battles, but the first one he records is in the fifth year (Alma 2), so the beginning point of the first year could not be because of a battle. Mormon sees these events as cumulative, and the great increase in detail we will see for not only the book of Alma, but also Helaman, where we have more information per year than any other section of the Book of Mormon save the sections on Christ’s visit, perhaps tell us that Mormon sees all of these events as critical to arriving at the point of Christ’s visit.