What is the battle of Armageddon?
The Battle of Armageddon is the great final war in which the Antichrist, the false prophet, and the armies of the world gather to fight against Christ, only to be utterly destroyed by the power of His spoken word when He returns as King of Kings and Lord of Lords.
The Battle of Armageddon takes place immediately after the Marriage Supper of the Lamb in heaven. While the redeemed are celebrating with Christ, the Antichrist has been preparing for this war ever since the sixth vial judgment dried up the river Euphrates to prepare the way for the kings of the east. Unclean spirits like frogs go out from the dragon, the beast, and the false prophet to gather the kings of the earth to battle at a place called in the Hebrew tongue Armageddon.
When the moment for war arrives, heaven opens and Jesus appears on a white horse, called Faithful and True, clothed in a vesture dipped in blood, with the name The Word of God written on Him. The armies of heaven follow Him on white horses, clothed in fine linen, white and clean. Out of His mouth goes a sharp sword, and He will rule the nations with a rod of iron and tread the winepress of the fierceness and wrath of Almighty God. Christ does not fight with any carnal weapon; His weapon is the sword of His mouth, His spoken word, and that alone is enough. At His voice, the flesh, eyes, and tongues of His enemies dissolve while they still stand on their feet, and confusion causes them to turn against one another.
The beast and the kings of the earth gather their armies to make war against Him that sits on the horse, but the beast is taken, and with him the false prophet, and both are cast alive into a lake of fire burning with brimstone. The remnant of the armies are slain with the sword that proceeds from His mouth, and the fowls of the air are filled with their flesh. The slaughter is so great that blood rises to the height of a horse's bridle and flows for about two hundred miles.
After the battle, an angel standing in the sun calls all the birds of the air to the supper of the great God, to eat the flesh of kings, captains, mighty men, horses and their riders, both free and bond, small and great. This is called the Supper of the Great God, or the Feast of the Birds, distinct from the Marriage Supper of the Lamb. At the end of this war, Jesus sets His feet on the Mount of Olives, the very place from which He ascended, and the mountain splits in two, forming a great valley. Because so much of the world's infrastructure would have been destroyed by the vial judgments, the warriors at Armageddon are left to fight with primitive weapons such as bows, arrows, swords, staves, and spears rather than modern arms.
The scriptures
And the sixth angel poured out his vial upon the great river Euphrates; and the water thereof was dried up, that the way of the kings of the east might be prepared. And I saw three unclean spirits like frogs come out of the mouth of the dragon, and out of the mouth of the beast, and out of the mouth of the false prophet. For they are the spirits of devils, working miracles, which go forth unto the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty. Behold, I come as a thief. Blessed is he that watcheth, and keepeth his garments, lest he walk naked, and they see his shame. And he gathered them together into a place called in the Hebrew tongue Armageddon.
And I saw heaven opened, and behold a white horse; and he that sat upon him was called Faithful and True, and in righteousness he doth judge and make war. His eyes were as a flame of fire, and on his head were many crowns; and he had a name written, that no man knew, but he himself. And he was clothed with a vesture dipped in blood: and his name is called The Word of God. And the armies which were in heaven followed him upon white horses, clothed in fine linen, white and clean. And out of his mouth goeth a sharp sword, that with it he should smite the nations: and he shall rule them with a rod of iron: and he treadeth the winepress of the fierceness and wrath of Almighty God. And he hath on his vesture and on his thigh a name written, KING OF KINGS, AND LORD OF LORDS. And I saw an angel standing in the sun; and he cried with a loud voice, saying to all the fowls that fly in the midst of heaven, Come and gather yourselves together unto the supper of the great God; That ye may eat the flesh of kings, and the flesh of captains, and the flesh of mighty men, and the flesh of horses, and of them that sit on them, and the flesh of all men, both free and bond, both small and great. And I saw the beast, and the kings of the earth, and their armies, gathered together to make war against him that sat on the horse, and against his army. And the beast was taken, and with him the false prophet that wrought miracles before him, with which he deceived them that had received the mark of the beast, and them that worshipped his image. These both were cast alive into a lake of fire burning with brimstone. And the remnant were slain with the sword of him that sat upon the horse, which sword proceeded out of his mouth: and all the fowls were filled with their flesh.
And the angel thrust in his sickle into the earth, and gathered the vine of the earth, and cast it into the great winepress of the wrath of God. And the winepress was trodden without the city, and blood came out of the winepress, even unto the horse bridles, by the space of a thousand and six hundred furlongs.
For I will gather all nations against Jerusalem to battle; and the city shall be taken, and the houses rifled, and the women ravished; and half of the city shall go forth into captivity, and the residue of the people shall not be cut off from the city. Then shall the LORD go forth, and fight against those nations, as when he fought in the day of battle. And his feet shall stand in that day upon the mount of Olives, which is before Jerusalem on the east, and the mount of Olives shall cleave in the midst thereof toward the east and toward the west, and there shall be a very great valley; and half of the mountain shall remove toward the north, and half of it toward the south.
And this shall be the plague wherewith the LORD will smite all the people that have fought against Jerusalem; Their flesh shall consume away while they stand upon their feet, and their eyes shall consume away in their holes, and their tongue shall consume away in their mouth. And it shall come to pass in that day, that a great tumult from the LORD shall be among them; and they shall lay hold every one on the hand of his neighbour, and his hand shall rise up against the hand of his neighbour.
And they that dwell in the cities of Israel shall go forth, and shall set on fire and burn the weapons, both the shields and the bucklers, the bows and the arrows, and the handstaves, and the spears, and they shall burn them with fire seven years: So that they shall take no wood out of the field, neither cut down any out of the forests; for they shall burn the weapons with fire: and they shall spoil those that spoiled them, and rob those that robbed them, saith the Lord GOD.
And, thou son of man, thus saith the Lord GOD; Speak unto every feathered fowl, and to every beast of the field, Assemble yourselves, and come; gather yourselves on every side to my sacrifice that I do sacrifice for you, even a great sacrifice upon the mountains of Israel, that ye may eat flesh, and drink blood. Ye shall eat the flesh of the mighty, and drink the blood of the princes of the earth, of rams, of lambs, and of goats, of bullocks, all of them fatlings of Bashan. And ye shall eat fat till ye be full, and drink blood till ye be drunken, of my sacrifice which I have sacrificed for you. Thus ye shall be filled at my table with horses and chariots, with mighty men, and with all men of war, saith the Lord GOD.
Proclaim ye this among the Gentiles; Prepare war, wake up the mighty men, let all the men of war draw near; let them come up: Beat your plowshares into swords, and your pruninghooks into spears: let the weak say, I am strong. Assemble yourselves, and come, all ye heathen, and gather yourselves together round about: thither cause thy mighty ones to come down, O LORD. Let the heathen be wakened, and come up to the valley of Jehoshaphat: for there will I sit to judge all the heathen round about. Put ye in the sickle, for the harvest is ripe: come, get you down; for the press is full, the fats overflow; for their wickedness is great. Multitudes, multitudes in the valley of decision: for the day of the LORD is near in the valley of decision. The sun and the moon shall be darkened, and the stars shall withdraw their shining. The LORD also shall roar out of Zion, and utter his voice from Jerusalem; and the heavens and the earth shall shake: but the LORD will be the hope of his people, and the strength of the children of Israel.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.