The Lord knew everything about the Assyrians, including their raging against Himself. Because they raged against Him and felt complacent about controlling their own destiny, He would teach them who was sovereign. He would lead them away as they had led prisoners they had taken captive in war, by putting hooks in their noses. Assyrian monuments picture this. As they directed the horses they took so much pride in, God would put a bit in their mouths and turn them back to their homeland. [source][source][source]
Isaiah next offered a sign to Hezekiah to assure him that God would indeed do what he had said. Compare the sign that God gave believing Hezekiah"s unbelieving father Ahaz ( Isaiah 7:14; cf. Isaiah 38:7; Exodus 3:12). [source][source][source]
"Some signs are aids to faith, like that in Isaiah 38:7. But others, like this one, aid later recognition that God was indeed at work." [1][source]