John Calvin Commentary Isaiah 9:11

"Therefore Jehovah will set up on high against him the adversaries of Rezin, and will stir up his enemies," — Isaiah 9:11 (ASV)

But the Lord will set up the adversaries of Rezin against him. Because the Israelites were proud of their alliance with the king of Syria and hoped that everything would succeed as they wished, Isaiah threatens a new change that will take away all their hope and completely frustrate their plans, for the Assyrians later took up arms and waged war against the Syrians. Accordingly, when Rezin had been slain (2 Kings 16:9), that country was entirely ruined. He heightens this even more by adding, he shall mingle. For by this, he means that the Lord will gather together and mingle various enemies whom he will commission to destroy the king of Syria, just as the soldiers who composed the army of that vast monarchy were gathered from various nations.