What does Judges 3:12 mean?

Earlier Scripture established the pattern to be followed by the rest of the book (Judges 2:6—3:6). Each cycle begins as shown here, demonstrating Israel's crippling faithlessness to the Lord God. Despite His faithfulness to the people, rescuing them time and again, they always end up back to doing evil in the sight of the Lord. Mostly, that evil takes the form of worshiping the gods of the Canaanites and forgetting Yahweh. While this verse doesn't specify, that seems to be the overall theme of Israel's sinful habits.

This time, the Lord will use the Moabites to punish His people. Moab was an incestuous son of Lot, Abraham's nephew (Genesis 19:36–37), so the Moabites were distantly related to Israel. God has specifically forbidden Israel from trying to take Moab's land, east of the Dead Sea, which He had provided for them (Deuteronomy 2:9).

Now, in fact, God will use Moab against Israel for their faithlessness to Him. He strengthens Eglon, the king of Moab, to give him the ability to defeat and enslave Israel.