Isaiah - Chapter 31
If Assyria Can't Stand the Heat...
Isaiah continues to criticize King Hoshea of the Northern Kingdom of Israel for relying on Egypt, rather than on G-d, for help against Assyria. G-d knows the thoughts and deeds of man and pays them back accordingly. G-d told the Jewish people not to go back to Egypt (see Deuteronomy 28:68); since they went there voluntarily, He will bring them there again against their will. The Egyptians are just people; they have no ability to help unless G-d wills it. All He has to do is "turn His hand" and they will fall.
G-d is like a lion, poised to attack its prey (Assyria). The shepherds who oppose such a lion are of no consequence to it. G-d will spare and protect Jerusalem.
Isaiah urges the people to return to G-d, Whom they have ignored. When G-d destroys the Assyrian army, the people will realize that their idols are powerless and will regret turning to them in the first place. Assyria will fall by supernatural means (we saw it in II Kings chapter 19 and we'll see it again soon, in Isaiah chapter 37). Sancheriv, king of Assyria will flee, seeking refuge because of G-d's miracle. G-d's "fire" is in the Temple altar and His "furnace" is in Jerusalem.