20 Now the people said unto Gidgiddoni: Pray unto the Lord, and let us go up upon the mountains and into the wilderness, that we may fall upon the robbers and destroy them in their own lands.
21 But Gidgiddoni saith unto them: The Lord forbid; for if we should go up against them the Lord would deliver us into their hands; therefore we will prepare ourselves in the center of our lands, and we will gather all our armies together, and we will not go against them, but we will wait till they shall come against us; therefore as the Lord liveth, if we do this he will deliver them into our hands.
The Book of Mormon has a very clear message with regard to just war theory. In particular, it is very stark in its depiction of offensive rather than defensive war. In particular, the declaration that if the Nephite armies were to go into the land of the robbers, they would be destroyed. Later the Nephites begin to be destroyed when they go on the offensive against the Lamanites.

On the other hand, in our day we have seen certain circumstances where offensive action may be morally justified or even required. Bombing the train lines leading to the gas chambers is a good example. It is wrong to be complicit and allow genocide and mass slaughter.

How do we reconcile these imperatives? Perhaps a partial solution can come from the Nephite decision to gather together. They did not leave vulnerable people in range of the robbers. Instead, they made sure those individuals were safe. And then they renamed on the defensive against aggression.

I definitely don’t have all the answers to these questions. Nor does the Book of Mormon provide simple or facile answers. I don’t believe the wars of today are identical to those ancient ones. But we can learn from the insistence on avoiding wars of aggression. And we can also learn from the unwillingness to simply leave vulnerable members to the slaughter.