The Great War of the Angels.

And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels,
And prevailed not; neither was their place found any more in heaven. 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. - Revelation 12:7-9