Jamieson, Fausset and Brown on Joel 3:9

9. The nations hostile to Israel are summoned by Jehovah to "come up" (this phrase is used because Jerusalem was on a hill) against Jerusalem, not that they may destroy it, but to be destroyed by the Lord (Eze 38:7-23; Zec 12:2-9; 14:2, 3).

Prepare war--literally, sanctify war: because the heathen always began war with religious ceremonies. The very phrase used of Babylon's preparations against Jerusalem (Jr 6:4) is now used of the final foes of Jerusalem. As Babylon was then desired by God to advance against her for her destruction, so now all her foes, of whom Babylon was the type, are desired to advance against her for their own destruction.