Then the sixth angel blew his trumpet, and I heard a voice from the four horns of the golden altar 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, the day, the month, and the year, were released to kill a third of mankind. The number of mounted troops was twice ten thousand times ten thousand; I heard their number. And this is how I saw the horses in my vision and those who rode them: they wore breastplates the color of fire and of sapphire and of sulfur, and the heads of the horses were like lions' heads, and fire and smoke and sulfur came out of their mouths. By these three plagues a third of mankind was killed, by the fire and smoke and sulfur coming out of their mouths. For the power of the horses is in their mouths and in their tails, for their tails are like serpents with heads, and by means of them they wound.
The rest of mankind, who were not killed by these plagues, did not repent of the works of their hands nor give up worshiping demons and idols of gold and silver and bronze and stone and wood, which cannot see or hear or walk, nor did they repent of their murders or their sorceries or their sexual immorality or their thefts. (ESV)
Sixth Trumpet - Second Woe
Horns of the Alter
They serve at a sanctuary that is a copy and shadow of what is in heaven. This is why Moses was warned when he was about to build the tabernacle: "See to it that you make everything according to the pattern shown you on the mountain." (Heb 8:5 NIV)
We are reminded several times in Scripture that Moses built everything in the tabernacle after the pattern that he had been shown. Ex 30:1-10 describes the altar of incense that was in the sanctuary on earth. It was small, only about 18 inches square and three feet high. It was overlaid with gold and must have represented this altar that is before the Lord. Twice a day the high priest was to offer incense on it. It appears that the heavenly version of this is where the angel offered the incense and prayers of the saints. Previously, we saw that the souls of martyrs were under it. It is from the horns on the ends of the altar that the voice commands the next woe to come. It impresses me that the Lord is the only one who can authorize these disasters, yet in this case it appears to be in response to the prayers and death of His saints that the Lord lets the second woe loose on mankind.
Four Angels Loosed
God bound these four angels for this very time. Because they are bound, I doubt that they were holy angels in willful obedience to God. Otherwise, like the angels in Rev 7:3, they would have simply been told to restrain themselves.
But the prince of the kingdom of Persia was withstanding me for twenty-one days; then behold, Michael, one of the chief princes, came to help me, for I had been left there with the kings of Persia. (Dan 10:13 NASU)
Then he said, "Do you understand why I came to you? But I shall now return to fight against the prince of Persia; so I am going forth, and behold, the prince of Greece is about to come.” (Dan 10:20 NASU)
In Daniel, God revealed that nations have spiritual princes that are not from God. They resist the efforts of the godly angels. Perhaps the angels that have been bound are powerful spiritual princes that will guide other nations into war. If this is true, can you imagine what kind of war will ensue when these four angels are released? The war involves 200 million troops and will wipe out a third of all mankind that is left on the earth.
Since all angels, even the demons have been created by God, it is significant that their purpose was prepared even before the creation of the world. Just as Jesus’ death was planned before the creation of the earth, these demons were created with the full knowledge of God that they would rebel and be bound for this exact time. When you think about this, it reveals God’s sovereignty in ways that we can never imagine.
Because these angels are bound at the river Euphrates, it is assumed by many that these troops will be coming from the East. In 1961, China reported that they had this many militiamen. If they were to mobilize at the direction of these demon princes, and come to the Euphrates though India, surely a third of the world’s population would be killed either as they tried to resist or even as the troops simply plunder along the way. We think that their intent is simply to invade the Holy Land; however we need to remember that they are led by demons and would therefore wreak havoc along the way. The area of the Euphrates has always been the center of spiritual problems. It is where the Garden of Eden was located and it is where the first sin occurred. Cane committed the first murder there. The tower of Babel was erected in this area, which was the forerunner of the city of Babylon with its spiritual decadence. Perhaps, even in their bound state, these angels have been able to influence mankind through the ages.
While this speculation seems to fit with modern possibilities, we need to remember that the forces that finally do enter the Holy Land will be from the north. This raises the possibility that these are not armies of men but armies of demons that are being unleashed upon the earth.
The description of the mounted troops, like the locusts, is very obscure, which leads to speculation that they are from China. At one time, I thought that it was clear they were people; however there is nothing in the description that would preclude demonic troops much as those depicted in the “Left Behind” series of books. Because they are numbered, I thought that it seemed more likely to be human beings rather than demons. However, the Greek for the number is not specific, as some translations would imply. Rather, it is made up of two words, both of which could be translated as myriads.
Dismurias or dis murias (dis-moor-ee-as'); an indefinite number of incalculable immensity; found only in Rev 9:16: a double myriad, twenty thousand.
Murias (moo-ree'-as); a ten-thousand; by extension, a "myriad" or indefinite number: 
This could have been translated, “The number of troops that I heard was so large that I have no way to express it other than it was a double myriad times a myriad.” This significantly reduces the probability that these are human forces.
In addition, the horses aren’t anything that we can identify with any certainty. When Native Americans saw the first trains, they called them iron horses. What John saw had about as much in common with a horse as a steam engine. The description of fire, smoke, and sulfur could be supernatural or it could relate to almost any kind of tank, rocket launcher, or vehicle used to launch a chemical war. This could explain John’s reference to plagues of fire, smoke, and sulfur. If this is true, then God is using mankind to destroy mankind. We also need to remember that God doesn’t need our help.
The breastplates of these troops are brilliant. They are decorated with fiery red, dark blue, and brilliant yellow. The last I saw, troops around the world are currently dressing in camouflage rather than drawing attention to themselves. Even the vehicles are designed to avoid detection. These troops must be either demonic or symbolic of the troops of many nations.
Ezekiel 23 describes what the Lord did to Israel and Judah because she had gone after other gods and put her trust in the powers of other nations and their gods. Israel was overcome by Assyria and Judah by Chaldea (Babylon). The interesting thing is that the princes and warriors of these nations are described as wearing either blue or red.
And they shall come against you With chariots, wagons, and war-horses, With a horde of people. They shall array against you Buckler, shield, and helmet all around. (Ezek 23:24 NKJV)
These who are dressed in red and blue came against Judah and there were hordes of them. It has already happened once when Babylon took over. In Revelation, the horsemen are wearing red, blue, and yellow. Perhaps what is happening is a combination of these nations coming against Israel once again rather than a Chinese invasion. Yellow isn’t mentioned but it could possibly represent the alliance of another country that is not mentioned in association with a color. Perhaps it is Egypt because they were also mentioned in the passages in Ezekiel.
The rest of mankind
God has been saying it for a long time now; people who do not repent will be destroyed. It is sad, but true. When God first gave His covenant to His people, He warned them and followed with quick punishment as in the following example.
And the Lord said unto Moses, The man shall be surely put to death: all the congregation shall stone him with stones without the camp. (Num 15:35 KJV)
Later, the Lord gave them longer and longer periods of time to repent before bringing disaster on the nation. It is similar to what we do with our children. When they are very young, quick discipline is needed to keep them from danger. As they grow older, we give them more and more latitude. Finally, when they are grown, we no longer punish them but society takes over. The rules of society are basically the same as the moral instructions we give our children. When society stops punishing immorality, it is left to God.
Whoever stubbornly refuses to accept criticism will suddenly be destroyed beyond recovery. (Prov 29:1NLT)
We may have many rebukes, but there is never a promise that this is the last one before we will be punished. We really don’t know when the last will come. Even though one third of mankind was killed, the remaining people don’t take the warning and refuse to repent.
Which of you will listen to this or pay close attention in time to come? Who handed Jacob over to become loot, and Israel to the plunderers? Was it not the Lord, against whom we have sinned? For they would not follow his ways; they did not obey his law. So he poured out on them his burning anger, the violence of war. It enveloped them in flames, yet they did not understand; it consumed them, but they did not take it to heart. (Isa 42:23-25 NIV)
Who will listen? We all belong to God; we are His creation and as such He has the absolute right to set the rules and declare what is the right way to live. He has the power to turn individuals or nations over to punishment when they don’t follow His ways. People want sin so badly that they ignore and reject the very words that will save them, not just from physical punishment in this life, but eternal punishment.
Or do you presume upon the riches of his kindness and forbearance and patience? Do you not know that God's kindness is meant to lead you to repentance? But by your hard and impenitent heart you are storing up wrath for yourself on the day of wrath when God's righteous judgment will be revealed. (Rom 2:4-5 RSV)
Do we sometimes get impatient with God, wanting Him to judge people because of their sin? It is a horrible thing that we are talking about, this idea of God’s wrath. These verses make it clear that there is a day of punishment when His wrath will be revealed. It isn’t just on the Day of Judgment, but also at other times in history. We may all face it the day we die, or in some other part of our life. It is only His kindness that keeps it from the whole earth right now. He wants everyone to repent, and is waiting. He will keep His promise.
When the final outpouring of His wrath comes, people will not repent as described in Revelation 9:20. Maybe He is waiting to bring final judgment until He knows that all who will repent have done so. Since the rest did not repent at this time, it is a solid indication that there will be very few who will believe and come to Him from this point on.
For as in those days which were before the flood they were eating and drinking, they were marrying and giving in marriage, until the day that Noah entered the ark, and they did not understand until the flood came and took them all away; so shall the coming of the Son of Man be. Then there shall be two men in the field; one will be taken, and one will be left. Two women will be grinding at the mill; one will be taken, and one will be left. (Matt 24:38-41NASB)
It is passages like Rev 9:20, 21 that bring out thoughts of the very end of the age. Jesus says that just like in the time of Noah, people will be carrying on doing what is normal with no regard for God, but more importantly, their sin will be increasing. God flooded the earth because of what they were doing in the time of Noah. Before the Roman Empire crashed, sexual sin was rampant and God judged them. Look at where we are today with unabashed homosexuality and open demon worship. Years ago this would not have been tolerated. Either this is a sign that the very end is coming or that we are at the end of an age for our society. Our present society cannot exist much longer in the direction it is going. However, no matter how bad things get, some will never respond. Salvation is available for all but not all will take it. These refuse it, at least at this time.
The rest of mankind did not repent. This doesn’t mean that there aren’t any Christians on earth at this time or that no more will come to Jesus. Revelation 11 discusses two witnesses that are on the earth at the end. Rev 13:7 and 10b lets us know that Christians will be on the earth when the anti-Christ rules. Rev 14:12-13 indicates that there will be Christians and even more people coming to Christ while other people take the mark of the beast. However, it will become harder and harder for Christians and even harder for those who do repent to make that decision.
But exhort one another every day, as long as it is called "today," that none of you may be hardened by the deceitfulness of sin … As it is said, "Today, if you hear his voice, do not harden your hearts as in the rebellion." (Heb 3:13, 15 ESV)
May you continue in your faith in Jesus and strive to bring others into this relationship before it is too late.
 (Biblesoft's New Exhaustive Strong's Numbers and Concordance with Expanded Greek-Hebrew Dictionary. Copyright © 1994, 2003, 2006 Biblesoft, Inc. and International Bible Translators, Inc.)