1 In the year that Tartan came unto Ashdod, (when Sargon the king of Assyria sent him,) and fought against Ashdod, and took it;
Sargon is well known to us (comparatively speaking) from other historical records.
2 At the same time spake the LORD by Isaiah the son of Amoz, saying, "Go and loose the sackcloth from off thy loins, and put off thy shoe from thy foot." And he did so, walking naked and barefoot.
Can you imagine? Suddenly this well-known prophet, who has been wearing sackcloth as a sign of mourning, is walking around town totally naked. I wonder what his wife thought of this.
If anyone tells you we should obey all the verses in the Bible, explain that, though all the Bible has lessons to teach, we are only to obey the ones God wants us to. He really doesn’t want everyone walking around naked.
3 And the LORD said, "Like as My servant Isaiah hath walked naked and barefoot three years for a sign and wonder upon Egypt and upon Ethiopia;
This nakedness is a sign to the people of Egypt and Ethiopia.
It was common practice when you conquered a country to take all the clothes the people owned, since, before mechanization, a change of clothes cost a good year’s salary, all clothes were very valuable. It also humbled and humiliated your enemy, showing how much stronger you were.
4 So shall the king of Assyria lead away the Egyptians prisoners, and the Ethiopians captives, young and old, naked and barefoot, even with their buttocks uncovered, to the shame of Egypt.
5 And they shall be afraid and ashamed of Ethiopia their expectation, and of Egypt their glory.
6 And the inhabitant of this isle shall say in that day, "Behold, such is our expectation, whither we flee for help to be delivered from the king of Assyria: and how shall we escape?"
Up until now, everyone in the region had been looking to Egypt (Ethiopia and Egypt are sometimes the same country, sometimes different. Both very powerful historically) for protection and salvation from Assyria since Egypt has really been the boss of the area since the time of Joseph. Now God is going to very visibly remove that hope.
It took some courage for Isaiah to obey this command, I'm sure.