Revelation 12, 7-8

Then war broke out in heaven; Michael and his angels battled against the dragon. The dragon and its angels fought back, but they did not prevail and there was no longer any place for them in heaven. 

Teaching

According to Father Pierre Martin de Viviès, P.S.S.:

If there is a biblical book where we encounter angels on almost every page, it is indeed the book of Revelation. Even if this book summons the Apocalypse in our mind, a catastrophic end of the world picture, this literature is first and foremost an encouraging prophecy. The text offers to its reader a series of visions encompassing time and space, and the announce of the final triumph of Christ over hostile powers strengthens our confidence. 

At the core of the Revelation book is the description of the war between Christ and Satan, in a grand vision associating the dragon, the woman and the Messiah (Revelation 12). The dragon's defeat leads to a war in heaven, loyal angels against rebel angels. Michael is the head of the faithful angels. His name is Hebrew, derived from the question מי כאל mī kāʼēl, meaning "Who is like God?"

This question, frequently encountered in the Psalms (70/71,19 for example), could be understood as war cry from this archangel, depicted under the traits of a fighter: "who pretends to be the equal of God?"

The angels' war leads to the expulsion of the dragon and his minions, who retreat on earth, much to the woe of its inhabitants. The rest of the book focuses on the inevitable defeat of the dragon, despite his attempts at controlling earth with the help of his acolytes, the beast of the sea and the beast of the land.

The book of Revelation ends with the vision of a new creation, purged from all evil and violence. At the core of this vision, the celestial Jerusalem is entirely illuminated by God's glory (Revelation 21 & 22). It is an angel who guides the seer, by explaining the vision to him. As the seer tries to prostrate himself at the angel's feet, the latter prevents him from doing so: "It is I, John, who heard and saw these things, and when I heard and saw them I fell down to worship at the feet of the angel who showed them to me. But he said to me, 'Don't! I am a fellow servant of yours and of your brothers the prophets and of those who keep the message of this book. Worship God.'" (Revelation 22, 8-9)

Prayer

O Glorious Archangel St. Michael, Prince of the heavenly host, be our defense in the terrible warfare which we carry on against Principalities and Powers, against the rulers of this world of darkness, spirits of evil. Come to the aid of man, whom God created immortal, made in His own image and likeness, and redeemed at a great price from the tyranny of the devil.  Fight this day the battle of the Lord, together with the holy angels, as already thou hast fought the leader of the proud angels, Lucifer, and his apostate host, who were powerless to resist thee, nor was there place for them any longer in heaven. That cruel, that ancient serpent, who is called the devil or Satan, who seduces the whole world, was cast into the abyss with his angels.

Behold, this primeval enemy and slayer of men has taken courage. Transformed into an angel of light, he wanders about with all the multitude of wicked spirits, invading the earth in order to blot out the name of God and of His Christ, to seize upon, slay, and cast into eternal perdition souls destined for the crown of glory. This wicked dragon pours out, as a most impure flood, the venom of his malice on men, of his depraved mind, corrupt heart, spirit of lying, impiety, blasphemy, his pestilent breath of impurity, and of every vice and iniquity. These most crafty enemies have filled and inebriated with gall and bitterness the Church, the Spouse of the Immaculate Lamb, and have laid impious hands on her most sacred possessions.  In the Holy Place itself, where has been set up the See of the most holy Peter and the Chair of Truth for the light of the world, they have raised the throne of their abominable impiety, with the iniquitous design that when the Pastor has been struck, the sheep may be scattered.

Arise then, O invincible Prince, bring help against the attacks of the lost spirits to the people of God, and give them the victory. They venerate thee as their protector and patron; in thee Holy Church glories as her defense against the malicious power of hell; to thee God has entrusted the souls of men to be established in heavenly beatitude. Oh, pray to the God of peace that He may put Satan under our feet, so far conquered that he may no longer be able to hold men in captivity and harm the Church. Offer our prayers in sight of the Most High, so that they may quickly conciliate the mercies of the Lord; and beating down the dragon, the ancient serpent who is the devil and Satan, do thou again make him captive in the abyss, that he may no longer seduce the nations. Amen.

By Pope Leo XIII, Vatican City, 1888