Rev 12:7-9
"Now war arose in heaven, Michael and his angels fighting against the dragon. And the dragon and his angels fought back, but he was defeated, and there was no longer any place for them in heaven. And the great dragon was thrown down, that ancient serpent, who is called the devil and Satan, the deceiver of the whole world."
The war in heaven: Michael vs. the dragon. Satan’s defeat in heaven is the result of Christ’s victory on the cross (cf. John 12:31 — 'now is the judgment of this world; now will the ruler of this world be cast out'). Satan’s fall from heaven is a consequence of Christ’s ascension.
Rev 12:11
"And they have conquered him by the blood of the Lamb and by the word of their testimony, for they loved not their lives even unto death."
The formula for Christian victory: the blood of the Lamb (Christ’s work) + the word of testimony (faithful proclamation) + not loving life even unto death (martyrial resolve). Victory over Satan is achieved not by military power, but by faithfulness to the Gospel.