What does Isaiah 13:7 mean?

As with who are arrogant in their self-reliance, facing one's own destruction at the hands of others is a shock. It's terrifying. It is not something those who dwell in that arrogance see coming. Isaiah is describing the massive armies that the Lord will bring together against Babylon at some future point in history. He will wield them as His weapons and obliterate the entire land of the Babylonians (Isaiah 13:3–5). The prophet has commanded these once-proud future rulers and people to "wail" at what will soon happen to them on the day of God's judgment (Isaiah 13:6).

The people of Babylon will act exactly as the Lord indicates here. The courage of their hearts will fail. Their hands will go limp. They see clearly in this moment that what they were relying on will blow away like dust. Their rulers, their gods, their power is simply nothing compared to the forces God has arrayed against them.