"Woe to those who go down to Egypt for help and rely upon horses, who trust in a multitude of chariots and in mighty riders! They have not turned to the Holy One of Israel and have not sought G-d. But He, too, is wise and will bring misfortune - He will not take back His word. He will rise against the house of the wicked and against the help of evildoers." - Isaiah 31:1-2
Despite Isaiah's rebuke of the previous chapter, the Jewish people did not heed him. So now, after they have already gone to Egypt, Isaiah just cries out "Woe!" They thought that G-d know not their thoughts and that He does not interfere in human affairs. No! Says Isaiah, G-d is "wise" to your thoughts and He will bring about the evil you fear - both you, the "wicked," and the Egyptians, the "evildoers," shall fall.
How foolish it is to trust in a human! - he cannot annul what G-d has decreed. How foolish that they were to place trust in Egypt - they would not keep their word as G-d keeps His. And Egypt's horses are only flesh without spirit and are just a tool in the Egyptian's hands - how can they help them more than Egypt itself? So when "G-d tilts His hand" which supported Egypt's prowess, they who leaned on Egypt shall also fall.
As all other creatures are no match for the lion, are G-d's creatures helpless before Him. Thus, the myriad of armies camped against Jerusalem are a meaningless show of strength before G-d. The plague which will strike the Assyrians will be as potent as a lion and as swift as a bird. Just as a bird flies in silence, will G-d save Jerusalem without war. Just as a bird spreads its wings to protect its young, will G-d protect Jerusalem from the plague which will strike the Assyrians.
"Return, Children of Israel, to Him from Whom you sought deeply to stray." - Isaiah 31:6
Repentance is relative to the sin, and the deeper the sin, the deeper must be the repentance. The Jewish people had been guilty of not trusting in G-d and thus needed to deeply mend their ways. So unlike his predecessors who sought outside help and did not trust in the help of G-d, King Chizkiyahu (Hezekiah) relied solely on the help of G-d and sought no human protection from Assyria. In fact, he surrendered all human effort and begged G-d to save them while they sleep.
Although Assyria shall fall through a plague and not by sword, they shall panic as if from a sword. Sancheiriv, himself, shall flee from sight, while his young soldiers are all "melting" from the fiery plague. The mighty and fearless Assyria shall be terrorized, even the general will fear as if from a "flag," from the front battle lines. G-d's furnace shall incinerate the nations, the "firewood," who had come to camp against Jerusalem.