What does Isaiah 21:9 mean?
Day and night Isaiah has faithfully occupied the watchtower. He is looking into the distance to see when the riders will come. Scholars for the most part assume this takes place during his vision from the Lord. In this vision he is watching and waiting for the enemy to ride up to Babylon's walls to begin the attack. Some commentators speculate that Isaiah may be calling out from Jerusalem's walls instead, looking for riders to approach with news from the battle for Babylon (Isaiah 21:6–8).In either interpretation, the horse riders have arrived, riding in pairs. Isaiah has either seen in his vision or received the news that Babylon has fallen. He adds that all the carved images of their gods have been shattered as well.
Many commentators believe Isaiah's prophecy is about Sennacherib's utter destruction of Babylon in 689 BC. For many years prior, Babylon's ruler Merodach-Baladan had resisted Assyria's attempts to put down his rebellion. Assyria was particularly determined to crush these rebellions, because Babylon's strength inspired revolts by cities and states around the region. Finally, Sennacherib succeeded in first laying siege to Babylon and then breaching its walls. We know from his own annals that he unleashed soldiers to fully obliterate the city. They killed all the inhabitants and smashed all the gods and idols—apart from the two they carried off with them. The Assyrians even rerouted the river to flow through the town and destroy its foundations.
In the end, though, it is the Lord who brought down Babylon. Sennacherib was just a tool of the Lord God. Assyria's defeat of Babylon destroyed the hope of many nations that Babylon would save them that threat. Many in Judah shared that hope, but the Lord didn't want His people to trust in other nations or their to rescue them.
Many years later, John's vision, recorded in Revelation 14:8, echoes Isaiah's words. This adds a prophecy about the fall of another Babylon. John hears an angel say, "Fallen, fallen is Babylon the great, she who made all nations drink the wine of the passion of her sexual immorality." That future Babylon will also come crashing down by the Lord's hand. God's people will see they don't need such worldly kingdoms to survive. They, as well as all of us, need only the Lord to save us.