Today references to the Kings of The East by modern Bible Prophecy speakers pretty much always assume they are the Far East, China, Korea, Japan, India and the other nations around them.
But The Biblical references to them during the Sixth Bowl account in Revelation 16:12-16 begins with talk about the Euphrates River being dried up. That river is depicted as the border holding them back.
The Euphrates river as a border should remind Biblically literate readers of God's original Covenant with Abraham, where it is the eastern most border of what God promised to Abraham.
But in John's day it was also the primary border between the Roman and Parthian Empires, whether you see John's day as Nero or Domitian's reign. With Parthia on the other side of that border were Parthian Client kingdoms, Edessa, Adiabene, Armenia, Media, Characene, Elymais, ect. Some of who were often caught in the middle of Rome and Parthia's ongoing Cold War.
You do not have to be Preterist to keep what these geographical terms in John's day meant in mind when trying to interpret Revelation. Keeping them in mind is just common sense. Problem for Preterists is the only Army to come from East of the Eurphrates to Judea during the 66-73 AD Jewish-Roman War was the army lead by Monobaz II which was on the Jews' side.
What intrigues me as a Futurist who is interested in the theory that History always repeats itself. Is how in many ways the way the world is lining up now is eerily similar to John's day, though with obvious differences too. The region of "Persia" being in conflict with the West is one example of that.
This emphasis on the Euphrates River I feel is also damaging to any Mystery Babylon theory that is dependent on making Babylon some non Mesopotamian region. Whether it's Rome, America, Jerusalem/Israel, Arabia, Turkey, whatever your fixation is. The name Babylon is not the only geographical tie Revelation has to that region.
The first Bowl of God's Wrath is the immediate aftermath of The Mark being instituted. The rest come in pairs. 2 and 3 are about the world's water supply being destroyed, 4 and 5 are about the Sun. So it's not a coincidence that 6 and 7 both mention names linked to Mesopotamia.
Satan, The Beast, and The False Prophet, use demonic entities to gather the world's armies but particularly it seems the Kings of the East to Armageddon against God. Marching on God is indeed his plan but I think he also has another intent here.
Remember, Babylon is NOT the Beast's Capital, whatever kind of Geo-Political entity she is, she wields power over The Beast, she is riding it. It hates her and wants her dead. I think he wants to lure the armies around Babylon away from her so he can do a sudden sneak attack on her.
You may be thinking "Problem with your theory is the Euphrates isn't a border anymore". Our official Maps still like to think of Iraq as all one country, but internally it's divided. Right now the Euphrates may not make a perfect border between it's internal divisions yet, but I think it's interesting that most of what is under Iranian backed Shiite control is East of the Euphrates.
If modern Iran winds up effectively controlling most of Iraq (besides what's Kurdish) east of the Euphrates. It will become an Empire that looks a lot like the ancient Parthian Empire.
And so that is a problem for people who back up their view of a Pre-Trib Gog and Magog invasion by saying that Iran seems to play no role in the events of the Final Week. In fact it does.
It's also possible that this could be how The Medes role in the fall of Babylon as recorded by Isaiah 13 and Jeremiah 50-51 ties into Revelation.