Book of Joel Chapter 3 Vs. 12

 The Lord Judges the Nations

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. Joel 3:12

Let the Heathen... That is, let the enemies of Christ and his church be aroused from that state of security in which they are, and prepare for their own defense. For in such a state the antichristian powers will be before their destruction (see Rev. 18:7).

and come up... Let them bestir themselves, and exert all the rigor and strength they have. Let them come in high spirits against the people of God; let them invade the holy land, and come even to the valley of Jehoshaphat. And, when come thither, let them, descend into the place appointed for their ruin.

The land of Judea being said to be higher than other countries, going to it is generally expressed by going up to it. Otherwise it is more usual to say that men go down to a valley than come up to it; and mention being made again of this valley, shows that the same thing is referred to here (as in Joel 3:2).

These words are said in answer to the petition (in Joel 3:11); for they are spoken by the Lord, as appears by what follows.

there will I... This not only refers to Armageddon, but to the judgment of the nations (Mat. 25:31-46). There gathered together from all parts. The allusion is to a judge upon the bench, sitting to hear and try causes, and pass a definitive sentence. And here it signifies the execution of that sentence; such a pleading the cause of his people, as to take vengeance and inflict just punishment upon their enemies (see Psm. 9:4).

This is the same battle where 2/3 of Gog in the land of Magog will die in battle. So many die in this battle, that people on the Mediterranean Sea must put clothes pins on their noses to stop the death odor. All Israel will be 7 months burying the dead.

Ezekiel 39:12 "And seven months shall the house of Israel be burying of them, that they may cleanse the land."

This plainly says that the deaths in this great battle are a judgment of God against the heathen people.

The Lord Himself now repeated the summons of the preceding verses, instructing the nations to enter the Valley of Jehoshaphat (cf. Joel 3:2). Employing agricultural imagery, He then commanded His warriors to destroy His enemies.