In our pre-mortal existence, one third part of the the hosts of Heaven were cast down with the adversary. Where are those “1/3rd hosts of Heaven?
Satan and the 1/3 part of the Hosts of Heaven that were thrust down, are here on this earth with us. John the Revelator tells us of their expulsion and destination in Revelation 12:7-9
7. And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels,
8. And prevailed not; neither was their place found any more in heaven.
9. And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.
The Prophet Gordon B. Hinckley further tells us; “That war, so bitter, so intense, has never ceased. It is the war between truth and error, between agency and compulsion, between the followers of Christ and those who have denied Him.” He goes on further to say; “In this work there must be commitment. There must be devotion. We are engaged in a great eternal struggle that concerns the very souls of the sons and daughters of God. We are not losing. We are winning. We will continue to win if we will be faithful and true.” (June 2007 Liahona)
So while the War in Heaven is still being waged here on this earth, so long as we stay true to our commitments and remain faithful, in the end, we will win.