Hence verse 12 was examined.
On first glance we could try to read this verse as saying that God would punish the king of Assyria. But on closer examination there are several phrases that form the whole picture especially when it is realized that the king of Assyria was already dead.Isa 10:12 wrote: Wherefore it shall come to pass, that when the Lord hath performed his whole work upon mount Zion and on Jerusalem, I will punish the fruit of the stout heart of the king of Assyria, and the glory of his high looks.
Here are the phrases to consider in a process :
king of Assyria
stout heart of the king of Assyria
fruit of the stout heart of the king of Assyria
1. King of Assyria
Why was the king of Assyria mentioned? It was cause he was the first of a sequence of empires and represented those empires. This then was the Assyrian, the Babylon, the Greek and the Roman empire. This is the essential reason Isaiah mentioned of the king of Assyria. God was addressing the political rule of these empires.
2. Stout heart of the king of Assyria
He ruled with pride and was placed into power to bring forth God's discipline of His people. But in pride, the Assyrian king went on to take over more countries than God prescribed for him. This is the "problem" of assigning earthly rulers to do tasks for God.
3. Fruit of the stout heart of the king of Assyria
The "fruit" of the pride of king of Assyria was to have several rulers and empires follow his example of ruling with pride and overuse of the power given by God. So the king of Babylon aquired the same pride. So did Alexander the Great.
God's punishment was against the "fruit of the stout heart of the king of Assyria" which meant that this succession of empires would be removed from its sort of political power. Such idea was also presented by the statue of Daniel 2, representing the political power of the sort held by these rulers and nations (Babylon, Mede-Persia, Greek, Roman).
When Christ came then He was starting to replace their political power with the kingdom of God as exercising influence over the nations, in a political sense. Jesus was the Rock cut out of the mountain without hands. (Dan 2:45) The Daniel 2 description of the dream then was a natural extension of the ideas presented in Isaiah 10 (and we should be observant of the earlier introduction of such ideas, the earlier prophecy followed by a later prophecy, given for clarification.)
(And the Isa 10:12 verse gets more interesting as its considered that the punishment of the world was to follow the destruction of Jerusalem.)