What does Isaiah 9:11 mean?
The people of Israel had become arrogant in Isaiah's day. They faced the upcoming onslaught of the Assyrians with the belief that they will simply rebuild bigger and better once it passed (Isaiah 9:10). They underestimated the Assyrians. But even more so, they gravely underestimated the Lord's judgment. It is God who is sending the Assyrians to wipe them out (Isaiah 8:7).Now Isaiah adds that the Lord is raising up the enemies of Rezin against them. Rezin was the king of Syria, north of Israel. Israel and Syria had an alliance. It is possible that Israel was trusting this alliance to give them protection from the worst attacks of their enemies. The problem, as Isaiah points out here, is that an alliance means that your ally's enemies become your enemies as well.
The Lord causes the enemies of Syria to turn their attention on Israel. This is in addition to the juggernaut of the Assyrian army. Israel will soon face destruction from a host of enemies.