Albert Barnes Commentary Isaiah 21:9

"and, behold, here cometh a troop of men, horsemen in pairs. And he answered and said, Fallen, fallen is Babylon; and all the graven images of her gods are broken unto the ground." — Isaiah 21:9 (ASV)

And, behold ... a chariot of men - This passage shows that the word ‘chariot’ (רכב rekeb) may denote something other than a wagon or carriage, since a chariot drawn by men cannot be intended. Perhaps the sense can be expressed by the word “riding”: ‘I see a riding of men approach;’ that is, I see “cavalry” drawing near, or men riding and hastening to the battle.

With a couple of horsemen - The word ‘with’ is not in the Hebrew. The meaning is, ‘I see a riding of men, or cavalry; and they come in pairs, or two abreast.’ A part of the sentence is to be supplied from Isaiah 21:7. He saw not only horsemen, but riders on donkeys and camels.

And he answered - That is, the watchman answered. The word ‘answer,’ in the Scriptures, often merely means to commence a discourse after an interval; to begin to speak (Job 3:2; Daniel 2:26; Acts 5:8).

Babylon is fallen - That is, her ruin is certain. Such a mighty army is drawing near, and they approach so well prepared for battle, that the ruin of Babylon is inevitable. The “repetition” of this declaration that ‘Babylon is fallen,’ denotes emphasis and certainty. :

For lo, thine enemies, O Lord,
For lo, thine enemies shall perish.

(Psalms 93:3):

The floods have lifted up, O Lord;
The floods have lifted up their waves.

A similar description is given of the fall of Babylon in Jeremiah 50:32 and Jeremiah 51:8. John has copied this description in the account of the overthrow of the mystical Babylon (Revelation 18:1–2).

Babylon was distinguished for its pride, arrogance, and haughtiness. It became, therefore, the emblem of all that is haughty. As such, it is used by John in the Apocalypse; and as such, it was a most striking emblem of the pride, arrogance, haughtiness, and oppression which have always been evinced by Papal Rome.

And all the graven images - Babylon was celebrated for its idolatry, and perhaps was the place where the worship of idols commenced. The principal god worshipped there was Belus, or Bel (see the note at Isaiah 46:1).

Are broken ... - That is, they shall be destroyed; or, in spite of its idols, the whole city would be ruined.