v1-17: God's judgment on the nations.
v1-3: The restoration of Judah is linked with the judgment of the nations. They are judged because of their treatment of "my inheritance, my people Israel." The Valley of Jehoshaphat (meaning 'the Lord judges'); see also v14, the valley of decision. Thus the Lord God makes a distinction, like a judge, drawing a line between sheep and goats. In 2 Chr 20.22-26 the enemies of the Lord were utterly destroyed.
v4-8: Tyre, Sidon, Philistia; specifically names as guilty of mistreating the people of Judah. Again, their judgment is linked with the restoration of Judah. Here is proof, if any were needed, of the Lord's jealousy and faithfulness. The Gentile nations will suffer in the same way that they made Judah suffer.
v9-11: The nations to be gathered for war, ploughshares beaten to make swords, reversing the prophesy of Mic 4.3.
v12-13: All the surrounding nations; those nearer Israel are more guilty than those far away. Even today, it is Arab nations around Israel that hold the greatest animosity towards Israel.
v14-16: The day of the Lord, as in chapter 1; a time of judgment on multitudes. The Lord will pronounce His judgment from Zion.
v17: Zion is the dwelling place of God, providing a place of protection for His own. As in 2.1, Zion is the centre of events. Ultimately (v21) the Lord dwells in Zion!
v18: The Valley of Acacias, a valley of abundant blessings.
v19: Egypt and Edom made desolate because of the violence they committed against Judah.
v20: Judah will be inhabited forever, whilst other nations will be destroyed.
v21: Matt 27.25; Israel now bears her guilt, but one day that will be forgiven, when the Lord dwells in Zion, amongst His forgiven people.