Chapters 13- 17 are prophesies about God's judgement of different lands. A prophesy is basically a God given prediction. But we think of a prediction as something that probably will happen, like a predicted thunderstorm. But God's predictions (prophesies) are for sure. But God in His loving grace and mercy is always giving the people of these lands one more chance. And that's why he sent the prophets.
Chapter 13 is God's judgment upon Babylon. Isaiah says the Babylon will never be inhabited, Nor will it be settled from generation to generation. But wild beasts of the desert will lie there, And their houses will be full of owls; Ostriches will dwell there, And wild goats will dance there. The hyenas will howl there. Chapter 14: Judgment to come on Assyria and the Philistines. Chapters 15 and 16: Judgement upon Moab. Chapter 17: Judgement against Syria and Israel.
Isaiah shifts gears a little in chapter 18. He warns the people of Judah not to accept any help from the Ethiopians. God knows that they will make an offer to Judah, but God tells the people of Judah that He is enough. Judah is fearful that Assyria will attack (and they will) so they are looking elsewhere for help.
Don't we do the same thing? God is our security, but don't we put more trust in things like money, popularity and material things than we do in God? God wants us to remember all the times that He was there for us. He is unchanging. If He did it before, He will be there for us again. Try this: sit very quietly and try to remember some of the hard stuff you went through. Then think about how it all worked out even though you thought it might not. God was there!
Isaiah continues with the prophesies of God's judgements. In Chapters 19, 21, 22, and 23, the judgements are prophesied against Egypt, Babylon, Edom, Arabia, Jerusalem and Tyre.
In chapter 20 The gives Isaiah a sign to act out. God told Isaiah to remove his outer garments and his sandals (basically strip down to his underwear) for three years as a sign to all of what will happen to Egypt and Ethiopia after the Assyrian invasion. Then the said, “Just as My servant Isaiah has walked naked and barefoot three years as a sign and a wonder against Egypt and Ethiopia, so shall the king of Assyria lead away the Egyptians as prisoners and the Ethiopians as captives, young and old, naked and barefoot, to the shame of Egypt.”