Albert Barnes Commentary Isaiah 18:6

"They shall be left together unto the ravenous birds of the mountains, and to the beasts of the earth; and the ravenous birds shall summer upon them, and all the beasts of the earth shall winter upon them." — Isaiah 18:6 (ASV)

They shall be left together — The figure of speech here is dropped, and the literal narration is resumed. The meaning is that the army will be slain and left unburied. Perhaps the “branches and twigs” in the previous verse denoted military leaders and the captains of the armies, who are now represented as becoming food for beasts of the field and for birds of prey.

To the fowls of the mountains — Their dead bodies will be unburied and will be prey for the birds that prey on flesh.

And to the beasts of the earth — The wild animals: the beasts of the forest.

And the fowls shall summer upon them — they will spend the summer there; that is, their bodies will continue to be unburied. And the beasts of the earth shall winter upon them. Their bodies will be unburied through the winter, probably indicating that they would provide food for the fowls and the wild beasts for a long time.

From the multitude of carcasses, these animals will find nourishment for a whole year; that is, they will spend the summer and the winter with them. It is perhaps not possible to determine when this was fulfilled, as we know so little about the circumstances of the people to whom it was spoken.

If this prophecy related, as I suppose, to the people of Nubia or Ethiopia forming an alliance with the Assyrians to invade Judea, it was probably fulfilled when Sennacherib and his assembled hosts were destroyed.

Whenever it was fulfilled, it is quite clear that the design of the prophecy was to give comfort to the Jews. They were alarmed and agitated by the prospect of the preparations being made. The prophecy assured them that those plans would fail and all the efforts of their enemies would be foiled and disconcerted.