Albert Barnes Commentary Isaiah 31:9

"And his rock shall pass away by reason of terror, and his princes shall be dismayed at the ensign, saith Jehovah, whose fire is in Zion, and his furnace in Jerusalem." — Isaiah 31:9 (ASV)

And he shall pass over - The margin reads, ‘His rock shall pass away for fear.’ The Hebrew could support this, but it does not convey a clear idea. The meaning seems to be this: The word translated ‘stronghold’ (Hebrew, ‘His rock’) refers to his fortifications, or the places of strength in which he trusted. The Assyrian monarch probably had many such places that he considered perfectly secure, both within the borders of his own kingdom and along his line of march toward Judea.

Those places would naturally be made strong to provide a refuge in case of defeat. The idea here is that his alarm at the sudden destruction of his army and the failure of his plans would be so great that in his flight he would “pass over” or “beyond” these strong places. He would not even stop to take refuge there and reorganize his scattered forces, but would flee with alarm “beyond” them and make his way to his own capital. This appears to have been most strikingly fulfilled .

And his princes - These were, perhaps, those who ruled over his dependent provinces.

Shall be afraid of the ensign - That is, of any standard or banner they saw. They would suppose it was the standard of an enemy. This indicates a state of great consternation, when all the princes and nobles under the command of the Assyrian would be completely dismayed.

Whose fire is in Zion ... - This means that His altar is there and always burns there. That was the place where He was worshipped, and it was a place, therefore, that He would defend. The meaning is that they would be as certainly destroyed as the God whose altar was in Jerusalem was a God of truth and would defend the place where He was worshipped.

And his furnace ... - (see the note at Isaiah 29:1). This is where His altar continually burns. The word translated ‘furnace’ (תנור tannûr) properly means a baking oven (Exodus 8:3; Leviticus 2:4; Leviticus 7:9; Leviticus 11:35). This was either a large conical pot that was heated, in which the cakes were baked on the sides, or an excavation made in the earth that was heated by putting wood in it; when the wood was removed, the dough was put inside.

Perhaps the whole idea here is that Yahweh had a home in Jerusalem, with the usual accessories of a house—that His fire and His oven were there, an expression describing a dwelling place. If so, then the meaning is that He would defend His own home, and the Assyrian could not expect to prevail against it.