After successfully defending themselves twice (with the Lord’s help), the Nephites began to “boast in their own strength” and decided to wage an offensive war for the first time in their history (see Mormon 3:9-10). Seeing that they had completely forgotten the Lord, Mormon refused to lead them any longer. Chapter 4 records the result of the Nephites’ fighting entirely in their own strength, and Mormon describes it as a “horrible scene” in which the Nephites were defeated. Within eight years of continual fighting and bloodshed, the forgetful Nephites had become the most wicked they had ever been (v. 12).
The Nephites made two mistakes here. First, they waged an offensive war. The bigger mistake was to not acknowledge the blessings of the Lord. It was for this bigger mistake that the Lamanites were allowed to prevail. I believe that, notwithstanding their wicked ways, had the Nephites turned to the Lord, they could have continued to progress as a people.
Let us not forget the Lord, but as Moroni will later teach us, let us remember His mercies toward us, and ponder them in our hearts (see Moroni 10:3).