The Church teaches that Jesus, indeed, will return in glory and that this world, as we know it, will come to a screeching halt. Yet this will not occur before a fierce, cosmic battle in which the enemy will make his final bid for world domination (Catechism of the Catholic Church, 675-677). Bringing the Era of Peace to an end, evil will once again find its way into human hearts, much in the mysterious way that Lucifer, once God’s powerful angel in heaven, His “Lightbearer,” somehow moved from great heights of holiness to an evil so dark that he convinced a third of the angels to join him in a doomed endeavor, which led them to everlasting hellfire.
The word, “Armageddon” is symbolic of this final of final confrontations, the last great battle of the ages between good and evil that will take place before the end of the world (Revelation 16:16). “Har” in Hebrew means mountain, and in Old Testament history, “Megiddo” was a place of numerous decisive battles because of the broad plain that stood before it. Deborah and Barak defeated Sisera and his Canaanite army there (Judges 4-5), Gideon drove off the Midianites and Amalekites (Judges 6), Saul and the army of Israel were defeated because of their failure to trust in God (1 Sam 31), and the Egyptian army under Pharaoh Neco killed Josiah, king of Judah (2 Kings 23:29).
We see hints of this final war in Revelation 16:14 and in Revelation 20:7-9, where Satan is unleashed through Revelation’s mysterious “God and Magog” and he gathers enemies from the four corners of the earth (in essence, everywhere).
Before the end of the thousand years the devil shall be loosed afresh and shall assemble all the pagan nations to make war against the holy city… “Then the last anger of God shall come upon the nations, and shall utterly destroy them” and the world shall go down in a great conflagration. —4th century Ecclesiastical writer, Lactantius, “The Divine Institutes”, The ante-Nicene Fathers, Vol 7, p. 211
They will surround the camp of the Christians, but fire from heaven will consume them:
When the thousand years are completed, Satan will be released from his prison. He will go out to deceive the nations at the four corners of the earth, Gog and Magog, to gather them for battle; their number is like the sand of the sea. They invaded the breadth of the earth and surrounded the camp of the holy ones and the beloved city. But fire came down from heaven and consumed them. The Devil who had led them astray was thrown into the pool of fire and sulfur, where the beast and the false prophet were. There they will be tormented day and night forever and ever. (Revelation 20: 7-9)
Then, the Catechism says:
The kingdom will be fulfilled, then, not by a historic triumph of the Church through a progressive ascendancy, but only by God's victory over the final unleashing of evil, which will cause his Bride to come down from heaven. God's triumph over the revolt of evil will take the form of the Last Judgment after the final cosmic upheaval of this passing world. —Catechism of the Catholic Church, n. 677