What does Isaiah 21:6 mean?
The powerful vision from the Lord has let Isaiah see and feel the destruction to come on Babylon. This reckoning is likely at the hands of the Assyrians in 689 BC (Isaiah 21:3–4). The prophet has also seen either the Babylonians, or his own people in Judah, going about their daily routines and perhaps even celebrating Babylon's seeming success against the Assyrians. Isaiah has called out to the leaders to prepare for battle (Isaiah 21:5).He now reports a message from the Lord calling for a watchman. This person is to be a lookout posted on the wall who makes regular reports about what he sees. It will become clear that it is Isaiah who answers the Lord's call. The prophet will stand looking for riders to bring news of the fall of Babylon.
This will all be terrible news for anyone in Judah hoping that an alliance with Babylon would save them from the wrath of the Assyrians. The Lord does not want His people to be dependent on other nations, or their gods, to save them. He wants them to trust in Him alone.