The Revelation of John culminates in a great battle. This is linked to wars between armies on earth, but that is just a sideshow. The real battle takes place in the spiritual realms and Jesus achieves a great victory. John saw into heaven.
I saw heaven standing open and there before me was a white horse, whose rider is called Faithful and True. With justice he judges and wages war. His eyes are like blazing fire, and on his head are many crowns. He has a name written on him that no one knows but he himself. He is dressed in a robe dipped in blood, and his name is the Word of God. The armies of heaven were following him (Rev 19:11-14).
John saw things that took place in the spiritual realms. Jesus was leading the armies of heaven. These are the hosts of angels that love to do his will. They are supported by the prayers of the saints on earth. John was told that Jesus is King of Kings and Lord of Lords (Rev 19:16).
An angel cried out in a loud voice to call the evil ruler spirits to battle.
I saw an angel standing in the sun, who cried in a loud voice to all the birds flying in midair, “Come, gather together for the great supper of God, so that you may eat the flesh of kings, generals, and the mighty, of horses and their riders, and the flesh of all people, free and slave, great and small (Rev 19:17-18).The kings, generals and mighty men with their armies are not human kings and armies. They are the parallel spiritual powers that control the governments, nations and armies of men. These spiritual kings and generals control large hosts of evil spirits. They stop fighting against each other and come together to fight against Jesus.
The birds of the air represent ordinary garden variety evil spirits. These are spirits attached to particular sins, rather than to rulers and nations. They know a battle was coming and they hope to gain a position of power in the spiritual realm, when other more powerful spirits fall.
The gathering of the spiritual kings and generals is described in several places in Revelation.
When the sixth trumpet was sounded, angels were released to gather armies near the Euphrates area in the Middle East (Rev 9:13-16). This river runs through modern Iraq and Syria. Human armies will gather in this place, but they are not so important. The armies that John saw being gathered were spiritual (Rev 9:17-10). They were destructive spiritual powers.
The same gathering of spiritual powers was described in the sixth bowl of wrath.
The sixth angel poured out his bowl on the great river Euphrates, and its water was dried up, so that the way of the kings from the east might be prepared. And I saw three unclean spirits like frogs coming out of the mouth of the dragon, out of the mouth of the beast, and out of the mouth of the false prophet. For they are spirits of demons, performing signs, which go out to the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty (Rev 16:12-14).Human armies gather near Israel to fight, but the real battle is in the spiritual realms. The human armies will be relatively small, compared with the vast host of evil spirits gather for the battle on the great day of God. The beast and the false prophet are leading spiritual powers of evil.
The background to these events is described in Times and Seasons.