Revelation 9:14-16 “Release the four angels who are bound at the great river Euphrates.” So the four angels, who had been prepared for the hour and day and month and year, were released to kill a third of mankind. Now the number of the army of the horsemen was two hundred million; I heard the number of them.

I remember hearing sermons in the late 70’s/early ’80s on this prophecy.
I read that dams were being built on the Euphrates. There are some twenty-two dams on the Euphrates/Tigris river systems.
These dams and the increased arid climate of the region are having a drying-up effect on the great river.
As for an army of 200,000,000, China is the only nation that can boast of that number.
They seem a good candidate for the army that crosses the Euphrates.
However, it could be a combination of several allied countries creating an army of two hundred million.
The army kills a third of mankind.
Revelation 9:18-19 says, by these three plagues a third of mankind was killed—by the fire and the smoke and the brimstone which came out of their mouths. For their power is in their mouth and in their tails; for their tails are like serpents, having heads; and with them they do harm.
Could this indicate some kind of biological warfare?
Look how many have succumbed to Coronavirus.
It doesn’t take a large weapon to wipe out life, it only takes an effective disease that is easily spread with no foreseeable cure.
One of the interesting things about this section is the lack of repentance.
Revelation 9:20-21 But the rest of mankind, who were not killed by these plagues, did not repent of the works of their hands, that they should not worship demons, and idols of gold, silver, brass, stone, and wood, which can neither see nor hear nor walk. And they did not repent of their murders or their sorceries or their sexual immorality or their thefts.
Despite Revelation revealing these sequence of events, people continue to ignore the Gospel and continue in sin.