Isaiah asks the corrupt people who are passing unjust laws and oppressing the widows and the orphans what they'll do when the time for judgment comes. He suggests that they won't be able to escape when everyone's getting killed and taken prisoner by invaders. God's anger is still very much "on."
But Assyria—which is doing most of the invading and plundering—is going to get its come-uppance too. First, however, Assyria is going to act as God's own fury, attacking the godless people who've failed to worship him.
Assyria doesn't know that it's serving God's purposes. It just wants to destroy, plunder, and cut off different nations. Assyria will ask why, if it has destroyed so many other nations and overthrown their idols and gods, it won't be able to do the same thing to Israel.
But God will destroy Assyria after it has finished its destructive work, punishing its king for his arrogant boasts.
God says that his own strength will accomplish all of this destruction, in reality. He's the one who's doing it. God gathered up all the wealth and kingdoms of everyone as though he were gathering eggs from a bird's nest. No one even could chirp against him.
God vs. Smoky the Bear
God asks whether an axe or a staff can rebel against the person who uses it. The answer (in case you were wondering) is "No." So, in the same way, God says he will wreak destruction on his own warriors, who were once his tool.
The light of Israel will become like a fire, and so will the Holy One himself. It will be like another forest-fire, burning away all the briers and thorns and trees.
When that happens, Israel finally will only have God to support it. It will have none of the other false things that it has been trying to lean on.
Only a remnant of the people will ultimately survive and become righteous.
Assyria Comes to a Serious End (Get It?)
Nonetheless, God tells Israel not to be afraid of the Assyrians when the attack happens. Just as the Egyptians' rule over the Hebrews came to an end, so will Assyria's.
Once they've accomplished his purpose (of destroying Israel), God will turn on the Assyrians and annihilate them too, just as he swallowed the Egyptians with the sea.
Then what follows is a long list of all the places through which God will wreak destruction on his way to shake his fist at Zion.
Continuing with the tree-chopping metaphor from earlier, Isaiah says God will cut down the mighty and proud, like a man chopping down really tall trees—like those in the forests of Lebanon.