The Lord issued a call to war. The nations will evidently believe that God is calling them to do battle, but, ironically, it is really to hear His sentence of judgment against them. The nations should prepare for a great battle by beating their plowshares into swords and their pruning hooks into spears. At a later time, in the Millennium, they would do the reverse because Messiah will end war (cf. Isaiah 2:4; Micah 4:3). The weak should "psych" themselves up in preparation. The nations should hurry and assemble (cf. Zechariah 12:9). Joel also called on Yahweh to bring down His mighty army of angelic warriors to engage the enemy of His people (cf. Deuteronomy 33:2-3; 2 Kings 6:17; Psalm 68:17; Psalm 103:19-20; Zechariah 14:5). [source][source][source]
"But, when the nations were assembled in the valley, fully equipped for battle, they would receive a shock; they would find there the Judge of all the nations, and in their hands they would be holding the incriminating evidence of their own history of violence." [1][source]