Joel 3:18

These promises with which this prophecy concludes have their accomplishments in part in the kingdom of grace, and the comforts and graces of all the faithful subjects of that kingdom, but will have their full accomplishment in the kingdom of glory; for, as to the Jewish church, we know not of any event concerning that which answers to the extent of these promises, and what instances of peace and prosperity they were blessed with, which they may be supposed to be a hyperbolical description of, they were but figures of better things reserved for us, that they in their best estate without us might not be made perfect. I. It is promised that the enemies of the church shall be vanquished and brought down, Joe 3:19. Egypt, that old enemy of Israel, and Edom, which had an inveterate enmity to Israel, derived from Esau, these shall be a desolation, a desolate wilderness, no more to be inhabited; they have become the people of God's curse; so the Idumeans were, Isa 34:5. No strength nor wealth of a nation is a defence against the judgment of God. But what is the quarrel God has with these potent kingdoms? It is for their violence against the children of Judah, and the injuries they had done them; see Eze 25:3, Eze 25:8, Eze 25:12, Eze 25:15; Eze 26:2. They had shed the innocent blood of the Jews that fled to them for shelter or were making their escape through their country.

— Matthew Henry's Concise Commentary (public domain)

mountains . . . drop . . . wine--figurative for abundance of vines, which were cultivated in terraces of earth between the rocks on the sides of the hills of Palestine (Amo 9:13). hills . . . flow with milk--that is, they shall abound in flocks and herds yielding milk plentifully, through the richness of the pastures. waters--the great desideratum for fertility in the parched East (Isa 30:25). fountain . . . of . . . house of . . . Lord . . . water . . . valley of Shittim--The blessings, temporal and spiritual, issuing from Jehovah's house at Jerusalem, shall extend even to Shittim, on the border between Moab and Israel, beyond Jordan (Num 25:1; Num 33:49; Jos 2:1; Mic 6:5). "Shittim" means "acacias," which grow only in arid regions: implying that even the arid desert shall be fertilized by the blessing from Jerusalem. So Eze 47:1-12 describes the waters issuing from the threshold of the house as flowing into the Dead Sea, and purifying it. Also in Zac 14:8 the waters flow on one side into the Mediterranean, on the other side into the Dead Sea, near which latter Shittim was situated (compare Psa 46:4; Rev 22:1).

— Jamieson-Fausset-Brown Commentary (public domain)