Mormon had assessed the situation and decided that only a defensive war would be effective against the “exceeding great power” of this new kind of army. (See commentary accompanying Morm. 2:3.) Nevertheless, the Nephites had decided to take the offensive. An offensive requires more men and certainly more effort than a defensive war. Mormon’s stronghold had the advantages of a defensible position and virtually unassailable supply lines; going on the attack against an enemy who had undoubtedly established a defensive perimeter, they could not have been on more than an equal footing and, as it turned out, were actually in a much weaker position. The Nephite defeat and rapid retreat weakened both the army and its defenses. The Lamanite/Gadianton army breached the narrow pass, making the natural barriers irrelevant.
From this point on, the Lamanite/Gadianton force that had “exceeding great power” is in more open territory and occupies cities in the northernmost parts of the Nephite lands.