John Calvin Commentary Isaiah 9:21

"Manasseh, Ephraim; and Ephraim, Manasseh; and they together shall be against Judah. For all this his anger is not turned away, but his hand is stretched out still." — Isaiah 9:21 (ASV)

Manasseh, Ephraim. These tribes were closely related to each other; for besides being descended from the same ancestor, Abraham, a close relationship arose from their descent from one patriarch, his grandson, Joseph. (Genesis 41:50–52) But though they were closely allied, God still threatens that he will cause them to destroy themselves by mutual conflict, as if they were devouring the flesh of their own arm. Consequently, there will be no need for foreign enemies. He likewise adds that, after having exhausted themselves with mutual wounds, both will unite against Judah to destroy it.

And for all this his anger shall not be turned away. If anyone takes into consideration those calamities which Isaiah threatened, he will be amazed and greatly astonished that still more severe chastisements are foretold. But in this manner God acts towards the wicked, and does not cease to afflict them until he utterly overwhelms and destroys them, when, after having been frequently invited, they refuse to be reconciled to him. We need not wonder, therefore, that he inflicts stroke after stroke, as he also foretold by Moses that he would punish seven times more (Leviticus 26:18), and bring seven times more plagues upon (Leviticus 26:21) those who would not repent, lest they should think that, when they had been punished once or twice, they would not be punished again.

But his hand is stretched out still. By this he means that rods are prepared, that he may immediately strike with them; for the Lord is not angry with a light and capricious passion, but his wrath is immediately followed by revenge.