What is Armageddon?
Answer: According to the Revelation to John in the New Testament, where the author has a vision in Chap 16:13-16: (13) And I saw three foul spirits like frogs coming from the mouth of the dragon, from the mouth of the beast, and from the mouth of the false prophet. (14) These are demonic spirits, performing signs, who go abroad to the kings of the whole world, to assemble them for battle on the great day of God the Almighty. (15) See, I am coming like a thief! Blessed is the one who stays awake and is clothed, not going about naked and exposed to shame. (16) And they assembled them at the place that in Hebrew is called Harmagedon (=Armageddon) (Rev 16:13-16).
Harmagedon, i.e. Mount Megido. This city on the plain, which borders on the Carmel mountain range, was the site of the defeat of King Josiah, 2 Kings 23:29f; it remained as a symbol of the army’s downfall that assembled here, cf. Zach 12:11. The gathering of the kings in Verse 14 refers to the assembling of all the heathen tribes in the face of their destruction by Christ.