The story here is straightforward, but the way it is told is convoluted. The basic information is that Coriantumr, a Nephite dissenter, leads a Lamanite army against the Nephites. We have seen this pattern before. However, we know nothing about Coriantumr or why the Lamanites might be attacking.
Mormon appears to realize that he has neglected to get some of the details straight and corrects that neglect in verses 16 and 17. The actions represented by those two verses would have occurred prior to what we see in verses 14 and 15, but Mormon was backfilling the information he realized he had missed. This suggests that, while Mormon had an outline of what he wanted to write, he was not copying from a previously written manuscript, but rather writing directly onto the plates.