In chapters one and two of Joel, all the difficulties that come upon God’s people are the result of their own sins. In other words, they have only themselves to blame. Their punishment is earthly and temporary, and it is brought about by the enemies of God. Though they are enemies and attacking God’s people, they are just doing God’s bidding.
But in chapter three, God brings these evil nations into the Valley of “Decision” (compare 3:2,14) where He judges them for doing His will.
How can that be fair?
The passage speaks to the absolute sovereignty of God. We might reply: “God didn’t actually tell Judah’s enemies to carry them away and enslave them. They did it, and God used that to punish His people.”
And we’d be right, but the text does not exactly say that. The entirety of the situation is pictured as being run by God and this speaks to His absolute sovereignty. God knew what the nations were going to do. He used their predisposition against His own people to discipline them. And then, God punished Judah’s enemies for the actions of their predisposition.
You should not fail to see one other point: God loves His people more than He loves those who are not His people. All the nations are disciplined according to God’s will, but God is only a refuge for Judah.