What does Isaiah 8:6 mean?

God has been speaking to Isaiah about coming destruction. This will be like a flooded river overpowering its banks and sweeping everything away (Isaiah 8:7). The "river" which comes is the invading army of the Assyrians. Their mighty empire is on the move, and the king of Assyria will bring its terror into their region. The Assyrians will first wipe out the enemies of Judah who have been threatening them, Israel and Syria. While that might seem good, at first, Assyria is not planning to stop. The Assyrians will continue to "flood" into new territory.

The prophecy in this passage is for the people of Judah. The Lord compares the power to save to the flowing waters of rivers. Ahaz and his people have refused the power of the Lord, represented here as moving waters. They did not believe the Lord's power would be effective in defending them against Israel and Syria. So instead they put their hope in the "river" of the king of Assyria (2 Kings 16:7–9).

In fact, by the time this part of Isaiah was written, Assyria had has already, or soon would, wipe out Israel and Syria. The people of Judah are rejoicing over this fact. The enemy they dreaded so deeply has been, or will soon be, destroyed. The king of Syria was Rezin and the son of Remaliah, Pekah, was king of Israel.

The next verses will reveal that the rejoicing of the people of Judah over the defeat of their enemies is premature. The powerful force they trusted to solve their problem will soon turn its strength against on them.