What does Isaiah 9:21 mean?

Wickedness is like a fire that grows and consumes all the fuel it can reach (Isaiah 9:1–19). It is also like cannibalism in that we devour each other and even ourselves to feed the insatiable appetite for more sin (Isaiah 9:20; Galatians 5:15).

Now Isaiah describes what this unchecked wickedness has done to Israel. Manasseh and Ephraim devour each other. These were two of the largest and most prominent tribes in Israel. Both were descended from two sons of Joseph. This heightened their connection as brothers among tribes of brothers. When sin goes unstopped, unrestrained, even brothers will destroy each other. That's what sin does (Psalm 50:19–20).

Isaiah's references to these conflicts between tribes in northern Israel reflects the endless coups and battles for the control of the throne of ancient Israel. United, the nation of Israel stood against their brothers in the nation of Judah. Inside the borders, however, the battles between the tribes could be just as intense. Each man, family, and tribe sought only to serve themselves.

Isaiah ends the chapter by repeating, for the third time, that despite all this judgment on Israel, the Lord's anger was still not satisfied. Without repentance, the Lord's hand remained outstretched and ready to strike His people again.