Moroni had reinforced the cities of Ammonihah and Noah to such an astonishing degree that the Battle at the City of Noah was over before it started. Moroni's diligent preparations, in a manner which had never been known among the children of Lehi (v. 8), had outwitted the enemy again.
In our personal fight against evil, the same principle applies. We, too, can win the battle before it even starts by preparing ourselves spiritually and making certain decisions before we are ever confronted with the opposition. How many times have our youth been taught that it is much easier to make the decision not to use drugs or alcohol long before the opportunity presents itself? So it is with the Word of Wisdom, the Law of Chastity, and many other principles.
Figuratively speaking, to win the battle before it even starts, we must build forts of security (v. 13). We must strengthen our weakest areas, knowing as the Lord has said, if they humble themselves before me, and have faith in me, then will I make weak things become strong unto them (Ether 12:27). We must dig deeper ditches, build taller banks, and reinforce them with our strongest defenses. "The Lamanite attack on the city of Ammonihah reminds us that, like an enemy army, Satan will quietly try to dig down our defenses when he is not allowed direct access to our hearts." (Kathleen S. McConkie, Ensign, Jan. 1992, "Defending Against Evil")
But if we arm ourselves with the armor of righteousness, preparing ourselves as Moroni did, we will be able to slay the devil and his angels with an immense slaughter (v. 21). There will still be a battle, for Satan will do all he can to tempt, destroy, and seduce. Inevitably, there will still be battle wounds, many of which [are] very severe (v. 24). But if the vital decisions and preparations have been made beforehand, we will not be slain by the fiery darts of the wicked one (DC 27:17). Instead we will thank the Lord [our] God, because of his matchless power in delivering [us] from the hands of [our] enemies (v. 28).