The Sixth Trumpet

The Sixth Trumpet
Two Hundred Million Mounted Troops
The sixth angel sounded his trumpet, and I heard a voice coming from the horns of the golden altar that is before God. It said to the sixth angel who had the trumpet, "Release the four angels who are bound at the great river Euphrates." And the four angels who had been kept ready for this very hour and day and month and year were released to kill a third of mankind. The number of the mounted troops was two hundred million. I heard their number.
 The horses and riders I saw in my vision looked like this: Their breastplates were fiery red, dark blue, and yellow as sulfur. The heads of the horses resembled the heads of lions, and out of their mouths came fire, smoke and sulfur. A third of mankind was killed by the three plagues of fire, smoke and sulfur that came out of their mouths. The power of the horses was in their mouths and in their tails; for their tails were like snakes, having heads with which they inflict injury.
The rest of mankind that were not killed by these plagues still did not repent of the work of their hands; they did not stop worshiping demons, and idols of gold, silver, bronze, stone and wood — idols that cannot see or hear or walk. Nor did they repent of their murders, their magic arts, their sexual immorality or their thefts.
Rev 9:13-21 NIV

The Sixth Trumpet is the second woe (terror and calamity). In the time of the Sixth Trumpet; precisely within the days of the prophesying of “the two witnesses” [two prophets] sent to Israel. Remember, the Fifth Trumpet was the first woe (9:12).

The four that are bound at the great River Euphrates are fallen angels. They had been in readiness for such hour in the appointed day, month, and year, in which they will be loosed to destroy a third of mankind.

Two 10 thousands of 10,000 is 200,000,000 (two-hundred million). The two hundred million mounted troops are referred to as “the army of the horsemen” (9:16). They are ‘horsemen’ (figurative)—human army.
This army will kill a third of the remaining people on the Earth with fire, brimstone and smoke. (Read the prophecy about Gog and Magog in Ezekiel 38 and 39).

At this point, after the “locusts” and the army of the river Euphrates, it’s relatively easy to see how the world could convince themselves the previous plagues and judgments were natural occurrences. But these judgments are clearly supernatural, and permitted by God.