"For thus hath the Lord said unto me, Go, set a watchman, let him declare what he seeth. And he saw a chariot with a couple of horsemen, a chariot of asses, and a chariot of camels; and he hearkened diligently with much heed: And he cried, A lion: My lord, I stand continually upon the watchtower in the daytime, and I am set in my ward whole nights: And, behold, here cometh a chariot of men, with a couple of horsemen. And he answered and said, Babylon is fallen, is fallen; and all the graven images of her gods he hath broken unto the ground." (Isaiah 21:6-9)
The Lord commands Isaiah to have someone watch for what the Lord may declare next. The Hebrew participle behind watchman is intensive. The idea is to post a watchman who will watch intently.
After some time, the watchman saw chariots yoked to various beasts. He saw the chariots of an army. The phrase he hearkened diligently with much heed is literally he gave attention with attentiveness, much attentiveness. This was no small matter to Isaiah and his companion. They were fixed upon seeing and declaring what God had to say.
The watchman cries out, "A lion!" Exactly what he saw or exactly what he meant by this exclamation is difficult to say. Perhaps he saw the enemies of Babylon as a lion. Perhaps he saw the primary symbol of Babylon (a lion) as an indicator of whom this prophecy is against. Regardless, he saw this after much waiting. The vision did not come quickly. Long days and wearisome nights of waiting preceded God's revelation.
When the vision did come, it likely made little sense to Isaiah and his friend. Not yet a world power, and subjugated numerous times by the kings of Assyria, Babylon is declared to have fallen. Unknown to Isaiah, Babylon would rise to dominance and almost as quickly as she arose would fade into the shadows of history. In this expression of the watchman, one can almost hear God's judgment against the future Babylon of the antichrist.
And there followed another angel, saying, Babylon is fallen, is fallen, that great city, because she made all nations drink of the wine of the wrath of her fornication (Revelation 14:8).
As with so many prophecies, the rule of double-reference may be applied here. God was definitely speaking of ancient Babylon; however, the future Babylon's judgment cannot be overlooked in such language.
On that terrifying night of God's divine retribution, Belshazzar's idols were conquered by the one true God. Throughout the years following Babylon's defeat, God would further bring to nothing her myriad of sorceries, and someday God will forever destroy both the city, heart and influence of Babylon which have continuously plagued mankind since the tower of Babel (Revelation 18).