Rev 9:13-21

THE BATTLE OF ARMAGEDDON
The sixth seal, trumpet and bowl of wrath all refer to the great battle prophesied in Dan 11:40-45, and Ezek 38-39.

Nowhere in these trumpets is another war prophesied. For, as Isaiah said, in the last days, the nations will beat their weapons into plowshares (Isa 2:4).  But when all men are saying “peace and safety” (1 Thess 5:3), the end will come with an army of 200 million men.
13 The sixth angel sounded. I heard a voice from the horns of the golden altar which is before God, 14 saying to the sixth angel who had one trumpet, “Free the four angels who are bound at the great river Euphrates.” 
Rev 9:14
Release the four angels bound at the great river Euphrates – in Rev 16:12, we are told that the sixth angel dries up the river Euphrates to make way for the kings from the East.  
15 The four angels were freed who had been prepared for that hour and day and month and year, so that they would kill one third of mankind. 
Rev 9:15
prepared for the hour, and day, and month and year – prepared by God, who alone “knows the day and hour” Matt 24:36

kill a third of mankind – this battle is not the end of man’s existence. The final destruction of man takes place when fire from heaven is sent down to destroy the earth. Here, the estimate of 1/3 is likely because the war takes place in a limited region of the world.
16 The number of the armies of the horsemen was two hundred million. I heard the number of them. 
Rev 9:16
200 million horsemen – in John’s time, this number was impossible. 200 years ago, this prophecy was also impossible. In 1800, there were only 1 billion people in the world. But today, a 200 million man army is completely possible.
17 Thus I saw the horses in the vision, and those who sat on them, having breastplates of fiery red, hyacinth blue, and sulfur yellow; and the heads of lions. Out of their mouths proceed fire, smoke, and sulfur. 18 By these three plagues were one third of mankind killed: by the fire, the smoke, and the sulfur, which proceeded out of their mouths. 19 For the power of the horses is in their mouths, and in their tails. For their tails are like serpents, and have heads, and with them they harm.
Rev 9:18
out of their mouths proceeds fire and brimstone – John’s description seems to resemble modern tanks, with their guns.

Rev 9:19
their tails are like serpents and have heads – possibly the rear gun of a tank or other military equipment
20 The rest of mankind, who were not killed with these plagues, didn’t repent of the works of their hands, that they wouldn’t worship demons, and the idols of gold, and of silver, and of brass, and of stone, and of wood; which can neither see, nor hear, nor walk. 21 They didn’t repent of their murders, nor of their sorceries, nor of their sexual immorality, nor of their thefts.
Rev 9:21
they did not repent – John calls these trumpets “plagues” and says that the people still would not repent, just as the Pharaoh of Egypt, would not repent. In spite of these plagues, the people continued their immorality. Here, we can really compare the deliverance of God’s people from Egypt to the final deliverance of His people from this world.