Don’t forget the circumstances of the book of Isaiah. Judah, the people of God, the Southern Kingdom, has become rebellious against the Lord. Politically, they’ve found themselves in deep trouble with their neighbors to the north.
But God, through Isaiah, assures His people that they should not fear the enemy they see (Israel and Syria). Instead, they should fear Him. God will deliver them from their neighbors, but that’s not a sign their troubles are over. God is bringing against them the Assyrians – a far more formidable foe. Assyria is the ‘rod of God’s anger,’ the ‘club of God’s wrath.’
The king of Assyria of course does not realize that he is the Lord’s puppet. He thinks he is simply conquering nations. But when the Lord is finished with him, the Lord will redirect His wrath to destroy the Assyrians (10:25-26).
The message of chapter 10 points to the sovereignty of God. He rules in the affairs of men to accomplish His will and nations are but tools in His hands to accomplish His purposes.