After he blew his trumpet the sixth angel was told to “Loose the four angels who are bound in the great river Euphrates” (Revelation 9:14). It seems that these angels held back something of import that, if not held in check, would spread across the whole of the land of the Jews prematurely. It is often thought that, because these angels are “bound,” they must be evil angels, but such is not necessarily so.
The Greek word used for bound (G1210) is also used of the marriage contract that binds a man and woman together as husband and wife (Romans 7:2; 1Corinthians 7:27, 39), and Paul used it to say he was bound in the spirit to go to Jerusalem (Acts 20:22). It was used by Mark to say a colt was tied in Bethany and held there for the Lord’s use (Mark 11:2-4). Finally, it is also used of the Lord telling the disciples that whatsoever they bind (G1210) on earth would also be bound (G1210) in heaven (Matthew 16:19; 18:18). So, just because these angels are bound (G1210) at the river Euphrates does not necessitate their being evil. It is possible that they had been singled out by the Lord to perform a specific service at a specific time, and this is what their binding means (Revelation 9:14).
The Euphrates River traditionally had been the boundary over which Israel’s enemies came from the north. In fact Syria, through which the Euphrates ran was referred to in Daniel as the King of the North (Daniel chapter 11; the King of the South was Grecian Egypt). When Rome conquered Syria, Rome became the King of the North, as far as Israel was concerned. In fact, Rome’s President of Syria was the Jewish state’s supreme authority. So, the Euphrates in Revelation 9:14 represents the boundary over which the Jews’ northern enemy would come.
The four angels were released at the river and had been prepared work to be done at a very specific time arranged by God (Revelation 9:15).From the very beginning the Lord knew Israel would become corrupt in her latter days (Deuteronomy 31:29). That is, they never considered the end to which their ways would lead (Deuteronomy 32:29), in that they were arrogant and a generation without faith (Deuteronomy 32:20). Therefore, the Lord called Israel his enemy and told the nations to rejoice over her, while he saved his servants from her evil works (Deuteronomy 32:41-43). The Lord saw this time afar off, even at the very birth of the nation of Israel. So, as one would prepare for a specific time of celebration, mourning or revenge, so the Lord did the same. However, at times the plans of men cannot be implemented, as they had desired, but this is not so with the Lord. He set the date millennia in advance, and kept it to the very hour (Deuteronomy 31:29; Revelation 9:15).
These four angels were prepared to slay men—many men, about a third of the whole nation of the Jews, whether part of the rebel force or the general public. A literal understanding of the number of the horsemen was 200,000,000 (Revelation 9:16). However, since the total force of Vespasian’s army was only sixty thousand,  and since the estimates of the total population of the entire Roman Empire was only a quarter of the former figure, it is difficult to believe John or any of his readers could possibly have visualized such a huge military force. 200,000,000 cavalry units, besides infantry, and all that belonging to one enemy is difficult to comprehend. As I mentioned in a previous study, the total combined military might of both sides in World War II was only one-third that figure, 70 million military units. Therefore, it seems this figure must be understood figuratively.
John says that he “heard the number of them” (Revelation 9:16), which may lead some to believe the figure should be taken literally. However, John also heard the number of the elect in Revelation 7:4, and nearly all Bible students agree that 144,000 is a figurative number. If we consider the Septuagint at Daniel 7:10 the Greek wording is similar to what we find concerning this great army in Revelation 9:16. Daniel says myriads of myriads or ten thousand times ten thousand (100,000,000) ministered to the Lord when the judgment books were opened.
So, when the appointed hour came for the Lord to judge his people, ten thousand times ten thousand (100,000,000) were at his disposal. What this means is this: the number 10, being one of the Lord’s perfect numbers, implies nothing is wanting in that it is complete. This number is then multiplied to the power of eight, which is the Lord’s number for abounding in strength.  Finally, in the context of Revelation 9:16, that product is multiplied by two indicating, opposition or enmity. Thus this army is the complete or whole army of God, abounding in might and sent as an adversary to judge his people, the Jews.
 Josephus: Wars of the Jews; 3.4.2
 See E.W. Bullinger: Number in Scripture; “Ten” – page 243.
 Ibid. “Eight” – page 196.
 Ibid. “Two” – page 92.