The Fall of Jerusalem - The Sixth Trumpet

 If the fifth trumpet is about the siege of Jerusalem, the sixth trumpet describes the fall of Jerusalem.

Revelation 9:13 Then the sixth angel blew his trumpet, and I heard a voice from the four horns of the gold altar that is before God.

The four horns are a reference to Zechariah 1:18

Zechariah 1:18 "Then I looked up and saw four horns. 19 I said to the messenger speaking with me, "What are these?" He said to me, "These are the horns that scattered Judah, Israel, and Jerusalem."

It means Israel is about to be scattered among the nations.

14 It said to the sixth angel, who had the trumpet, "Release the four angels who are bound at the great river Euphrates." 15 Then the four angels who had been made ready for that hour, day, month, and year were released to kill a third of men.

In the Vision of the Four Horses we have discussed that these four angels are the four horses from the first four seals in chapter 6.

16 The number of cavalry troops was two hundred million. I heard their number.

This number does not fit the number of soldiers of the Roman legions that besieged Jerusalem. Yet we have to take that number very serious, because it was not a lucky guess of John, but he says it had been told to him. Therefore it is possible that this prophesy has two layers or fulfillments. There is only one occasion during the history of mankind where two hundred million soldiers might have been involved and that is WWII.
John in the first place has seen the destruction of Jerusalem. But he saw another very similar event as well during the Holocaust of WWII.

17 And this is the way I saw the horses and their riders in the vision: they had breastplates that were fiery red, dark blue, and yellow as sulfur. The horses’ heads were like lions’ heads, and out of their mouths came fire, smoke, and sulfur. 18 By these three plagues a third part of men was killed: by the fire, smoke, and sulfur coming out of their mouths. 19 The horses’ power is in their mouths and their tails, for their tails are like snakes with heads that inflict injuries.

The description of the 'horses' and colors typically fit Roman soldiers. Their armor is in red and yellow and the metal of their breastplates is blueish.

The image of a lion with a snake as tail typically comes from Greek mythology, are called chimera and symbolize the god of war:

The Romans had images of animals like that on their banners.

The Roman legions also had siege weapons with which they could send fire unto a city.

The Jewish historian Josephus writes that 1 million Jews were killed during the fall of Jerusalem (although some people think this is a too high estimate). That might have been one third of all the existing Jews at that time.

During WWII Hitler killed about 6 million Jews from the existing 17 million. That's also one third.

Many people think these trumpets still have to happen and address the whole world population. But all the indications in the text show that this is about the people of Israel and it has already happened.

Another indication that the one third of people being killed cannot apply to the whole world population is that the four horses are only given power over one fourth of the earth (7:8).