What does Isaiah 9:12 mean?

Isaiah described the misplaced confidence of the people of the northern ten tribes of Israel. They are proud and arrogant about their own self-reliance. As well as convinced in their ability to survive and rebuild following any attack from outside enemies. They do not trust the Lord or turn to Him for help. Instead, they trust themselves and their allies, the Syrians. The Syrians were also known as Arameans (Isaiah 9:9–11).

Because of this arrogance, the Lord will multiply Israel's enemies to include the enemies of the Syrians. Those enemies would include other Syrians from their divided nation. These would come from the east. Israel's old enemies the Philistines, would become their enemies once more and attack from the west. Between them, these two nations will devour Israel. They will eagerly and greedily defeat Israel's forces and destroy its people.

At that point (Isaiah 9:9), the people of Israel will finally understand that they cannot overcome what is happening to them. They will come to know that they are being judged by the Lord for their faithlessness to Him. That judgment will not yet be at an end, because the Lord's anger has not been satisfied. He still has work to do to make Israel's judgment complete.