War in Heaven
This term arises out of Rev. 12:7 and refers to the conflict that took place in the premortal existence among the spirit children of God. The war was primarily over how and in what manner the plan of salvation would be administered to the forthcoming human family upon the earth. The issues involved such things as agency, how to gain salvation, and who should be the Redeemer. The war broke out because one-third of the spirits refused to accept the appointment of Jesus Christ as the Savior. Such a refusal was a rebellion against the Father’s plan of redemption. It was evident that if given agency, some persons would fall short of complete salvation; Lucifer and his followers wanted salvation to come automatically to all who passed through mortality, without regard to individual preference, agency, or voluntary dedication (see Isa. 14:12–20; Luke 10:18; Rev. 12:4–13; D&C 29:36–38; Moses 4:1–4). The spirits who thus rebelled and persisted were thrust out of heaven and cast down to the earth without mortal bodies, “and thus came the devil and his angels” (D&C 29:37; see also Rev. 12:9; Abr. 3:24–28).The warfare is continued in mortality in the conflict between right and wrong, between the gospel and false principles, etc. The same contestants and the same issues are doing battle, and the same salvation is at stake.
Although one-third of the spirits became devils, the remaining two-thirds were not all equally valiant, there being every degree of devotion to Christ and the Father among them. The most diligent were chosen to be rulers in the kingdom (Abr. 3:22–23). The nature of the conflict, however, is such that there could be no neutrals, then or now (Matt. 12:30; 1 Ne. 14:10; Alma 5:38–40).
From: War in Heaven
I believe that by understanding the War in Heaven, we can better understand the conflict between good and evil that is still going on in our mortal world.