The Hebrew word Satan means the adversary, the opponent / opposer or to withstand against.
The Old Testament often depicts Satan as an antagonistic character or an adversary in the judgment of mankind on earth. He was allowed to enter into The Eden Garden to tempt and deceive Eve into disobeying. Satan was disputing with Michael the Arch Angel over the body of Moses.
Satan was allowed to play a part in judging Job. He was allowed to provoke King David and stand up against Israel in 1Ch 21:1. In Zec 3:2 Satan was interfering with Joshua. Satan had even entered into Judas Iscariot.
We find that Satan was interfering with Jesus, as he tried to persuade and tempt Jesus in various things.
So Satan had always been here on earth working as an adversary challenging and judging against mankind. But what
Rev 12 is saying that a great war happened in heaven because when Christ was born, then the Dragon / Satan began to attack the woman, Israel and her seed which was Christ.
:4 And his tail drew the third part of the stars of heaven, and did cast them to the earth: and the dragon stood before the woman which was ready to be delivered, for to devour her child as soon as it was born.
This Greek word { DREW } means to call someone to hail others to come to you or to drag a net to catch fish or game. It is used in Joh 21:, Act 8:, Act 14: and Act 17: to describe as someone gathering others and calling them to gather or come to you. Also, as we see in Joh 21: where peter drew or dragged the net to catch fish.
: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 bible specifically shows that this war takes place exactly at the time after Christ.
Rev 12:10 And I heard a loud voice saying in heaven, Now is come salvation, and strength, and the kingdom of our God, and the power of his Christ: for the accuser of our brethren is cast down, which accused them before our God day and night.
And see, I never knew this until I read the bible. I did not know a lot of things until one day I just opened the book and read it.