Revelation 12:7-12
(Rev 12:7 KJV)
And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels,

When the Lord Jesus Christ went to the cross and rose the third day, the final war between God and Satan was set in motion. On earth, the believers began to spread the word of God until we get into our present time where the whole world now has the Gospel. It started out in Jerusalem, then went to Africa, and then to Europe, and fanned out to all the rest of the countries of the world until they all have a Gospel witness in them. Satan was unable to stop the spread of Christianity upon the earth because he was bound at the cross. The war in heaven was a response to the events on earth that the Gospel was now being sent forth and Satan had tried to do everything he could to stop it both on earth and in heaven.

(Rev 12:8 KJV)
And prevailed not; neither was their place found any more in heaven.

The result of the heavenly war is that Satan and his followers lost and were expelled from heaven. This would give us the reason why Satan has so much hatred toward the church. Satan once had an exalted position in heaven but now he has lost it and he can no more reenter heaven.

(Rev 12:9 KJV)
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.

Here we have a direct identification of who the dragon is. Satan has now lost all entrances to heaven and he has been cast to the earth where we are told that he deceives the whole earth. This would be spiritual deception where Satan would keep those in false religions under spiritual bondage. However, he does not have the authority to prevent anyone from becoming saved. Those whom God has chosen will become saved at their appointed time and all the deceptions that Satan can muster will not affect that timing.
(Mat 24:24 KJV) For there shall arise false Christs, and false prophets, and shall show great signs and wonders; insomuch that, if it were possible, they shall deceive the very elect. The Elect of God can never be so deceived that the light of the truth of the Gospel cannot penetrate it and save them.

(Rev 12:10 KJV)
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.

After the war where Satan tried to conquer heaven with his demons, his expulsion from heaven is now being lauded with a great voice. It is not clear if this voice is one person or if it is one uniform voice of all the inhabitants of heaven. It is probably the latter but is the unified voice of all the Christians in heaven. If you notice that Satan is called the accuser of the brethren. We do not find in Scripture where Satan brings accusations against angels. Satanís hatred is for the redeemed body of Christ, which he is not allowed to touch spiritually.

(Job 2:4-5 KJV) And Satan answered the LORD, and said, Skin for skin, yea, all that a man hath will he give for his life. {5} But put forth thine hand now, and touch his bone and his flesh, and he will curse thee to thy face.

(Zec 3:1 KJV) And he showed me Joshua the high priest standing before the angel of the LORD, and Satan standing at his right hand to resist him.

Satan has always mounted a campaign of false accusations against the brethren, but now in heaven he has been expelled because of the blood of Christ washing the believers pure, there is nothing which Satan can accuse the believers of. Christ completely cleansed His children and God looks at them as if they had never sinned.

(Rom 8:33-34 KJV) Who shall lay any thing to the charge of God's elect? It is God that justifieth. {34} Who is he that condemneth? It is Christ that died, yea rather, that is risen again, who is even at the right hand of God, who also maketh intercession for us.

(Rev 12:11 KJV)
And they overcame him by the blood of the Lamb, and by the word of their testimony; and they loved not their lives unto the death.

Here is the reason for the lauding and shouting in heaven. As humans we can never defeat the devil because of the strength and cunning he possesses but we are victors through the cross of Christ. It is only be being washed in the blood of the Lamb whereby we may obtain the spiritual victory over Satan. Satan was and is also overcome by the faithful witness of the brethren who continue to preach the true Gospel and they do it without fear of losing their physical lives.
(Mat 10:39 KJV) He that findeth his life shall lose it: and he that loseth his life for my sake shall find it. When a Christian is committed to the propagation of the true Gospel, are finding their life in Christ and are losing their life toward the world. They are detaching themselves from the world by remaining as faithful witnesses. The faithful witnesses who lose their life on earth by means of dedicating it totally for the service of the Lord in truth are in essence finding their life. It must be remembered that the true life that Christians will have is not here on earth but in Heaven. We read the lives of many who refused to compromise the true Gospel for a few more years here on earth. We read of Christians who were tortured and murdered by Rome, and then the Roman Catholic Church became Satanís killing machine in the middle ages and in these latter days false gospels and false versions are attempting to neutralize the sting of the Gospel.

(Rev 12:12 KJV)
Therefore rejoice, ye heavens, and ye that dwell in them. Woe to the inhabiters of the earth and of the sea! for the devil is come down unto you, having great wrath, because he knoweth that he hath but a short time.

We see here that because of Satanís expulsion from heaven that those in Heaven can now rejoice because evil has been expelled forever. Then we are seeing that a woe, which is grief or distress, will come upon the inhabitants of the earth because now the spiritual war is taking place on earth. We saw in verse 4, the great strength that Satan has and now that evilness is now being felt on the earth. Satan is also aware that his time is limited. This little time would probably be from the time of his expulsion until the final judgment when he will be cast into the lake of fire. Since we are not specifically told when this expulsion took place, there seems to be an imminence to the return of the Lord. This means that since only the Father knows the day and hour of the Lordís return, Satan will attempt to bring as much havoc upon the church as he possibly can in the short time that he has. An application here also can be made to the following verse.
(Mark 13:20 KJV) And except that the Lord had shortened those days, no flesh should be saved: but for the elect's sake, whom he hath chosen, he hath shortened the days. Satan has caused many Christians to be killed through false religions and evil governments and if he was allowed to go for thousands of years, then what would happen is that he could mount a campaign which would physically rid the earth of every Christian. This is why the Lord has shortened those days so there would still be many Elect left on earth when he returned.