Isa 20 is a conclusion of the burden against Eithiopia (Isa 18) and Egypt (Isa 19). Israel and Judah were looking to Egypt and Eithiopia to save them from Assyria. Egypt was ruled by an Eithiopian dynasty for a time, and the two nations were linked together for an epoch. God was assuring Israel and Judah, that man could not deliver anyone from Assyria, the rod of His judgment that He had established.
Isa 21 is a vision which Isaiah saw, of watchmen seeing the assembling of a great army. Elam is told to go up, and Media to besiege (Isa 21:2), and the result, is that Babylon is fallen (Isa 21:9). Elam is a country just to the west of Babylon and Chaldea (which modern day Iraq encompasses) in the current territory of modern day Iran. Elam was founded by Shem’s oldest son Elam and was a Semitic country of great power in Abraham’s lifetime, and still a significant power during this epoch of the kings of Israel and Judah.
Elam was sometimes just outside of Assyrian dominance, and sometimes joined with Babylon as Babylon was constantly struggling for independence from Assyrian rule. Media was a nation descended from Japheth’s son Madai, and was north of Elam. All three of these nations were at this time subject to Assyria. But Babylon, together with Media, eventually attacked Nineveh and prevailed, and in her turn became mistress of Asia.
As the nation of Elam had been devastated by an Assyrian monarch before this came to pass, when the nation recovered its strength, it was now known in history as Persia. However, Persia was in the same province and composed of the same people as Elam had been since the dispersal from Babel.
In exchange for Media’s help against Nineveh, Babylonia and Media had agreed to share the power equally should they prevail against Assyria. But it did not take long for Babylonia to forget this promise and impose lordship over Media as well. So Media, in indignation, began to look about her for allies against Babylonian rule, and made a marriage alliance with her neighbor to the south, Persia, the old Elam.
The Persians and Medians thus eventually joined together to destroy Babylon, an event many years in the future at the time Isaiah delivered this prophecy. But it came to pass exactly as the LORD had spoken through His servant the prophet.