Mormon gives us the moral of the story. The Nephites are not valiant in their righteousness, therefore they are losing. The proof of their unrighteousness is that they are losing. We recognize that this thinking does not take into account the effects of agency, but we may forgive Moroni and his chief captains for their assumptions. They have just had the miraculous preservation of the Ammonite youths, and they surely assumed that the same blessings should have been available to all. That they were losing ground had a certain logic in blaming those loses on the people’s lack of sufficient righteousness.