Alma 43:46 - 43:50
Alma 43: 46 And they were doing that which they felt was the duty which they owed to their God; for the Lord had said unto them, and also unto their fathers, that: Inasmuch as ye are not guilty of the first offense, neither the second, ye shall not suffer yourselves to be slain by the hands of your enemies.
The Nephites felt that it was just and right according to the scriptures that said that as long as one was not guilty of the first or second offence, it was right to defend oneself and their families against attacking enemies.
It is interesting that all battles took place on Nephite territory, not Lamanite territory. Nephites were always in defense posture and not in offensive mode as the Lamanites always were. All fighting was instigated by attacking Lamanite armies not Nephite armies attacking Lamanite territory. The only time the Nephites did attack Lamanite territory was under Mormon and he refused to lead them and they were soundly beaten.
Alma 43: 47 And again, the Lord has said that: Ye shall defend your families even unto bloodshed. Therefore for this cause were the Nephites contending with the Lamanites, to defend themselves, and their families, and their lands, their country, and their rights, and their religion.
The scriptures said that it was ok to kill the enemy to protect themselves and their families. This was the bases for their defending themselves with force of arms. In addition, they were defending their civil rights, freedom of religion and their personal property.
This verse is obviously place here by Mormon to justify the use of deadly force against the Lamanites. One might think it is a natural thing to do, that is, defend oneself with force, but Mormon must thought it was necessary to make this statement for our benefit.
Alma 43: 48 And it came to pass that when the men of Moroni saw the fierceness and the anger of the Lamanites, they were about to shrink and flee from them. And Moroni, perceiving their intent, sent forth and inspired their hearts with these thoughts—yea, the thoughts of their lands, their liberty, yea, their freedom from bondage.
With their way of escape blocked by the various forces of Moroni, the Lamanites began to fight with even greater vigor for they knew that they were dead men unless they fought harder. This in turn frighten the Nephites and they were about to flee from them. However, Moroni called out to his men to remember why they were fighting in the first place, for their freedoms and their families.
Alma 43: 49 And it came to pass that they turned upon the Lamanites, and they cried with one voice unto the Lord their God, for their liberty and their freedom from bondage.
With these words of encouragement from Moroni and the other leaders, the Nephites increase their vigor by calling out loud for God’s help to help them to defend their rights and families.
Alma 43: 50 And they began to stand against the Lamanites with power; and in that selfsame hour that they cried unto the Lord for their freedom, the Lamanites began to flee before them; and they fled even to the waters of Sidon.
This turned the direction of the fighting and the Lamanites were again being killed in large numbers and began to flee back to the river.