Cush was south of Egypt, a powerful enough kingdom to take control of Egypt more than once in Egypt’s history. At the time of this prophecy they were in control of Egypt.
Egypt’s downfall was unimaginable because it had always been a world power. Apart from the Chinese, Egypt was the longest-lasting civilization in the world, stretching from around 3100 B.C. to just before Jesus’ life on earth when the Romans conquered it and drained its wealth and glory. That’s a history measured not centuries but in millennia! Remember that in Abraham’s day Egypt offered a refuge of abundance and stability when Canaan suffered famine; it had been a place to which God’s people looked whenever they were in trouble. History reveals that an Assyrian king did defeat Egypt and take the throne in 671 B.C. Notice that God didn’t foretell Egypt’s destruction, but a downfall, a plague, and their turning to Him so that He can heal them. Of course we know that Egypt has waxed and waned, but has always existed. Isn’t it amazing that after all the history with Egypt, God wanted to heal them instead of destroy them?
The prophecy of the downfall of Egypt probably made the prophecy about the breaching of Jerusalem’s walls believable to some of Isaiah’s readers, but Judah was still counting on God’s presence in their midst inside the Most Holy Place of the Temple, to protect them. They failed to comprehend that their Temple worship based on the Law given through Moses, was not going to shield them from destruction.
To the first readers of this book, if God truly accomplished what He said He would do in these prophesies, He would be doing astounding acts. History shows that He did them. Let’s not forget that God is capable of astounding acts even at the international level.