Then the sixth angel sounded: And I heard a voice from the four horns of the golden altar which is before God,
saying to the sixth angel who had the trumpet, “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.
And thus I saw the horses in the vision: those who sat on them had breastplates of fiery red, hyacinth blue, and sulfur yellow; and the heads of the horses were like the heads of lions; and out of their mouths came fire, smoke, and brimstone.
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.
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 sorcery's or their sexual immorality or their thefts. 9:13-21
Since we are not told otherwise it is safe to assume that the voice from the altar might be the angel we met in 8:3, the one who stood at the altar offering the incense with the prayers of the saints.
Of the first five, in this group of angels, we read only that they sounded the trumpet, but of the sixth angel, we read that he is to do something more than sound the trumpet. Release the four mighty demons bound at the great river Euphrates. The Living Bible
It is the sixth trumpet blast that inaugurates the Battle of Armageddon; it heralds the second of three woes.
These four angels, bound at the Euphrates, are destroying angels, and it sounds as if perhaps they have four different plans; that is, they are ready for any contingency. They are ready to do battle for as long as it takes, whether it is an hour, a day, a month or even a year. However, Wuest# changes that meaning to; they had been kept in readiness for that particular time, down to the exact hour in which they would be loosed. Their mission was to kill a third of mankind.
Till this point in the Apocalypse the first five angels (with the five bowls of wrath), have destroyed thirty-three percent of the trees, The Sea, the creatures and ships of The (Mediterranean) Sea, and the apparent functioning of the sun, the moon, the stars, the day and the night. During the last three woes, death to humans happens on a large scale.
Maybe much of The Apocalypse is history to us. The four angels, from the Euphrates, might, as some suggest, represent the four major kingdoms of that area, namely: Babylon, Persia, Greece, and Rome. The locusts of the fifth sounding angel, 9:3-11, may refer to the Parthian cavalry, which rode against Rome, as is suggested by Asimov.
However, I think that I believe that this whole prophecy is still future for John. I think that these four angels who are bound at the great river Euphrates are four leaders of four armies coming from the East to The Mediterranean Sea area to wage the Battle of Armageddon.
The number of (equestrians) horsemen in the army is two hundred million, which is about six times as many as the total population of Canada. John adds I am not just guessing at the number, I heard the number of them. John says I saw the horses, (mode of transportation), on a video. We must not confuse the army of 9:7 with the army of 19:11-15. It is a completely different army with different horses.
Those who sat on them had breastplates of fiery red, hyacinth blue, and sulfur yellow. Unger++ says that in this case, jacinth (hyacinth, American Standard) refers to blue sapphire. Unger also says, “brimstone is the gum of the gopher tree and from there the meaning was transferred to all inflammable substances, especially sulphur”. In Greek, the word is theion, which means flashing.
Could it be that the fire, the flashing, and the blue intense heat refer to modern day explosive warfare? After all, these were not actual horses, for they had heads like lions, and it was out of those mouths that the fire, smoke, and flashings came. 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.
John says their power is in their mouths and in their tails. Perhaps he is referring to warfare with tanks. They fire forward and backward. Also notice that their tails are like serpents, long and skinny like the barrel of a tank, and the tails had heads on them. The flange at the end of a cannon barrel makes it appear as if it has a head, and, John says, it is with these heads that they do harm. The breastplates are the cockpits of a tank.
The Signs^^, suggests that the fifth angel’s trumpet depicts the battle between the Islamic Arabs and the Christian world and that the sixth trumpet blast continues with representations of the Islamic Turks. It's fire, smoke and brimstone may well represent their use of gunpowder, which enabled them to do what the Arabs hadn't been able to do - to capture Constantinople.
Here we come to the sad part of the story. Sixty-six percent of humanity is still alive but they repented not. Here they have another chance to straighten things out with Jehovah! Do they repent? No! They still worship devils and idols. They still kill, practice sorcery, commit fornication and steal.
++ Unger’s Bible Dictionary, Merrill F. Unger
^^ Signs of the Times Magaizines: What the Revelation says about. March 1998. David C. James