Once the Nephite soldiers started to wage offensive war they soon became so bloodthirsty they were concerned only with the taking of human life. Thus both the Nephites and Lamanites became obsessed with the desire to kill. Concerning this situation Mormon writes:
And it is impossible for the tongue to describe, or for man to write a perfect description of the horrible scene of the blood and carnage which was among the people, both of the Nephites and of the Lamanites; and every heart was hardened, so that they delighted in the shedding of blood continually.
And there never had been so great wickedness among all the children of Lehi, nor even among all the house of Israel, according to the words of the Lord, as was among this people. (Mormon 4:11-12.)
The leaders of this dispensation have also warned against the dangers of starting an offensive war. President Charles W. Penrose has said:
Now if a nation essays to go forth against another nation for the purpose of conquest, to gain territory, to grasp something that does not belong to that nation, then the nation thus assailed has the right to resist even to the shedding of blood, as it was in this land in the war for independence. But we have to be careful as to what spirit we are guided by. If we want to go out to battle, to encroach upon other peoples' liberties and rights, to gain their lands, to destroy their property without any right or reason, that is one thing; but if somebody comes against us to destroy us and our property and our homes and our rights and our privileges, either on land or sea, then we have the right under the divine law to rise for our own protection and take such steps as are necessary. But, . . . we Latter-day Saints must watch ourselves and not give way to passion and desire to shed blood and to destroy, for that is the power of the evil one. We do not want to imitate any nation that is bent on a policy of destruction, to destroy where they cannot rule, to break down and trample under foot where they cannot dominate. If we have that desire, it is the spirit of the wicked one. . . .
There is a very great difference between arising to go forth for conquest, for blood, for plunder, to gain territory and power in the earth, and in fighting to defend our own possessions in the spirit of justice and righteousness and equity, and standing up like men for those things that we have a right to contend for. (Conference Report, April 1917, pp. 21-22.)
(For further information concerning the teachings of modern prophets on this subject, read the material listed after 3 Nephi 3:20-21.)