Book of Joel Chapter 3 Vs. 11

 The Lord Judges the Nations

Assemble yourselves, and come, all ye heathen, and gather yourselves together round about: thither cause thy mighty ones to come down, O LORD. Joel 3:11

Assemble yourselves, and... From different parts into one place:

come, all ye... Anti-Christian nations, Mohammed or the Catholic Church: Which latter, especially, are sometimes called Heathen and Gentiles, because of the Heathenish rites introduced into their worship (Psalm 10:16). From all parts, to the valley of Jehoshaphat or Armageddon (Rev. 16:14). This is spoken ironically to them, to use their utmost endeavors to get most powerful armies against the people of God, which would be of no avail, but issue in their own destruction.

and gather yourselves... Be ye gathered; or gather yourselves together, as the Targum and Kimchi. Get together in a body, muster up all the forces you can collect together.

Jarchi, from Menachem, by the change of a letter, renders it, make ye haste. Don't lose time in preparing for this battle; get men, and arms for them, as fast as you can. Be as expeditious as possible.

Or it may signify what should be done by the providence of God, bringing such large numbers of them together to their own ruin.

thither cause thy... Which is a prayer of the prophet, or of God's people, to God, that he would send down his mighty ones, the angels that excel in strength, and destroy this great army thus gathered together. As an angel in one night destroyed the army of Sennacherib.

So Kimchi and Aben Ezra interpret it of angels, and many other interpreters. But perhaps it may be better to understand it of Christian princes and their forces, those armies clothed in white, and riding on white horses, in token of victory; with Christ at the head of them (Revelation 19:14) as well as the angels of God.

Who may be said to be caused to "come down"; because, being assembled shall go down into the valley of Jehoshaphat, where their enemies are gathered together, and discomfit them.

The Targum is, "there the Lord shall, break the strength of their strong ones."

They are rushing to their doom. The mighty ones could come to battle, but they are no match for God. God, Himself, fights for Israel.

And they are assemble for battle (Joel 3:11; cf. Zec. 12:9). The Lord is urged to bring down His warriors.