3:18 Joel continued to describe the future day of the Lord, but now he passed from the judgments of the Tribulation to the blessings of the Millennium. The mountains of Israel would be so full of grapevines that they could be described as dripping with wine. There will be so many milk-yielding animals feeding on the luxuriant hills that the hills could be said to flow with milk. Instead of the wadis that have water in them only a few days each year, the steams of Judah would flow with abundant, life-giving water. All these descriptions recall conditions in paradise (cf. 1:5, 18, 20). A spring will flow out from the millennial temple that will water the valley of acacia trees, evidently between Jerusalem and the Dead Sea (cf. Ezek. 47:1-12; Zech. 14:8). This will also be a visual reminder that Yahweh is the source of all provisions and fruitfulness.
3:19-20 Egypt and Edom, probably representative of Israel's enemies, will become deserts because they shed innocent blood, presumably the blood of God's people. But Judah and Jerusalem would be full of people for all generations to come (cf. Ezek. 37:25; Amos 9:15; Zech. 14:11).
3:21 God's final promise through Joel was that He would avenge the blood shed by these enemies of Israel, which He had not yet avenged in the prophet's day. He promised to do this because He dwelt in Zion, that is, He had a special covenant relationship with Israel (cf. Ezek. 43:1-12; Zech. 2:10-13).
The prophecy of Joel unfolds in chronological sequence. It begins with reference to a severe locust invasion that came as a judgment on the Judahites for their covenant unfaithfulness to Yahweh (1:2-20). Even though it is impossible to date this plague, it happened in the recent past from Joel's perspective. The Lord used this severe judgment to call His people, through His prophet, to anticipate an even worse devastation coming in the near future, not from insects but from foreign invaders. He called on the Jews to repent and promised that if they did He would forgive them and spare them from this invasion. This would be a day of deliverance in which they would learn that He was at work for them. Yet another similar day was coming farther in the future in which they would again experience an invasion by foreigners who hated them. Nevertheless Yahweh promised to deliver them in that day and to restore them to unprecedented blessing because He was their covenant God.
