Mormon had assessed the situation and decided that only a defensive war would allow the Nephites to prevail against the “exceeding great power” of this new kind of army (see Mormon 2:3 and commentary following). Nevertheless, the Nephites had decided to take the offensive. An offensive tends to require more men, and certainly more effort, than a defensive war. With the advantages of defensible position and virtually unassailable supply lines of Mormon’s last defensive stronghold, the Nephites were able to withstand.
However, when they had to turn the tables and attack an enemy position that certainly would have had some defensive perimeter, they were on equal footing at best, and at worst (and it seems worst was the case) in a much worse position. The defeat of the Nephite army created the need for rapid retreat, and weakened both the army and the nature of the defenses. Significantly, the Lamanite/Gadianton robber army is now through the narrow pass, and that natural barrier that was a protection once from the north, and now from the south is no longer a factor.
From this point on, the Lamanite/Gadianton force that had “exceeding great power” is once again in more open territory, and occupies cities in the most northern parts of the Nephite lands.