These verses conclude the recapitulation of the historical situation behind this new crisis. In verse 15, Mormon is finished with the background and returns to the current topic he started in verse 5. Interestingly, verse 5 began the story of the armies being gathered together in the land of Antionum, and then led to the discussion of the Lamanite leader.
Here, this same beginning is used, and the next text introduces the Nephite leader (see the verses that follow).