Book of Joel Chapter 3 Vs. 2

 The Lord Judges the Nations

I will also gather all nations, and will bring them down into the valley of Jehoshaphat, and will plead with them there for my people and for my heritage Israel, whom they have scattered among the nations, and parted my land. Joel 3:2

I will also... At the same time God will gather the nations for judgment.

Bring them down... The site of the judgment will be the Valley of Jehoshaphat, mentioned only in Joel 3:2, 3:12. Whether such a geographical site was known by this name in ancient Israel is not certain. Some scholars suggest it is a yet-future valley, to be formed by the splitting of the Mount of Olives at the Messiah’s return (Zec. 14:4). At any rate, the importance of the name is not in its geographical location, but in its meaning, “the Lord judges.”

valley of Jehoshaphat... The name means “Yahweh judges” (3:12, 14), and although the exact location is unknown, other prophets spoke of this judgment as occurring near Jerusalem (Ezek. 38, 39; Dan. 11:45; Zech. 9:14; 12:1). This judgment of the nations includes the event of (Matt. 25:31-46).

I believe this is speaking of the great battle of Armageddon. There has never been a time in history before, when all nations were gathered in battle here in Israel. This is the same as the battle we dealt with (in the 39th chapter of Ezekiel). God fights the battle for Israel in this valley of Jehoshaphat.

Israel was promised this land back in Abraham's time. It was also promised that there would be a King that would rule on the throne in Jerusalem, who would be like unto David. The physical house of Israel is, even now, returning to the land of Israel. They have been scattered in all the countries of the world. God's plan all along was to restore Israel.

and will plead... The reason for God’s judgment is the nations’ treatment of His covenant people (My inheritance; see Joel 2:17), My people. The nations had scattered the Lord’s people, sold them as slaves to distant lands, and divided up His land.

The site of the judgment will be the Valley of Jehoshaphat, mentioned only in Joel 3:2, 3:12. Whether such a geographical site was known by this name in ancient Israel is not certain. Some scholars suggest it is a yet-future valley, to be formed by the splitting of the Mount of Olives at the Messiah’s return (Zec. 14:4). At any rate, the importance of the name is not in its geographical location, but in its meaning, “the Lord judges.”