After Mormon had led his armies to victory in at least three major encounters, the Nephites
began to boast in their own strength, and began to swear before the heavens that they would avenge themselves of the blood of their brethren who had been slain by their enemies.And they did swear by the heavens, and also by the throne of God, that they would go up to battle against their enemies, and would cut them off from the face of the land. (Mormon 3:9-10.)
When the Nephite soldiers decided to attack the Lamanites first, Mormon refused to lead them in offensive war. This position was not only justified by earlier teachings of the prophets and the counsel of the Lord (see 3 Nephi 3:20-21; Alma 43:46; Alma 48:14), but was also vindicated by subsequent events: the Nephite armies began to be defeated from that time forth. (For further information on the teachings of the Book of Mormon concerning war, read the material listed after 3 Nephi 3:20-21.)