21:6, 7 Watchmen on the city walls often appear in prophetic visions of destruction. They are the first to see trouble coming. The prophet Habakkuk was a watchman (Habakkuk 2:1). The vision of the chariots and warriors could represent the Medes and Persians attacking Babylon in 539 B.C.
21:8, 9 Babylon was not only a great and powerful city, it was also filled with horrible sin (idolatry, witchcraft, and temple prostitution). Babylon was, and remains, a symbol of all that stands against God. Despite all its glory and power, Babylon would be destroyed, along with all its idols. They would give no help in time of trouble.
21:10 Threshing and winnowing were two steps in ancient Israel’s farming process. The heads of wheat (often used to symbolize Israel) were first trampled to break open the seeds and expose the valued grain inside (threshing). The seeds were then thrown into the air, and the worthless chaff blew away while the grain fell back to the ground (winnowing). Israel would experience this same kind of process: The sinful, rebellious people (worthless chaff) would be taken away, but God would keep the good “grain” to replenish Israel.
21:11 Dumah, or Edom, had been a constant enemy of God’s people. It rejoiced when Israel fell to the Assyrians, and this sealed Edom’s doom (34:8ff; 63:4). Seir was another name for Edom because the hill country of Seir was given to Esau and his descendants (see Joshua 24:4). Obadiah foretells, in great detail, the destruction of Edom.
21:13ff The places listed here are all in Arabia. They are border cities that controlled the trade routes through the land. Nabonidus, king of Babylon, would attack Arabia and make the people his servants. He set up court in Tema, leaving Belshazzar in Babylon as regent. This is Isaiah’s prediction of disaster. Judah kept trying to make alliances with Arabia against Nabonidus, but Isaiah warned the people against such an alliance and urged them to trust in God alone.