New Defender's Study Bible Notes
9:14 Euphrates. These four angels are also Satanic angels, for they have been “bound” because of past sin. They have not, like the previous group, been bound in the bottomless pit, but “in the great river Euphrates,” so apparently their particular sin was at a different time and place. Perhaps this particular horde of fallen angels, with their four captains, had been associated with the first great human rebellion after the Flood, when Nimrod led mankind to rebel against God at Babel, located on the Euphrates. As a result, God had scattered the people around the world, confusing their tongues (Genesis 11:9).
The invisible host of heaven, however, who had instigated this rebellion and whom Nimrod had sought to worship in his great temple tower built “unto heaven” (Genesis 11:4)—that is, with a shrine dedicated to the host of heaven, the angels and their starry realms—had not been scattered. Rather, they were confined to the Euphrates where they had established their base. This, of course, is not stated in Scripture, but seems a plausible explanation of why Babylon and its environs (including Baghdad, which has been more prominent than Babylon during the church age, as a center of the Islamic rebellion against Christ), seems ever since to have been the greatest enemy of God and His people.
9:15 a year. The demonic locusts had tortured men for only five months, but these demonic horses will ravage mankind for thirteen months, making a total of eighteen months. The previous judgments of the six seals and four trumpets will presumably take about two years, so that the events under this sixth trumpet will evidently terminate at about the midpoint of the seven-year tribulation.
9:15 third part of men. Evidently around a billion people will be slain by the Satanic army. Men could not die, though they desired to do so, under the torment of the previous scorpion-like stings; now they could hardly escape death. Already a billion people had been slain under the fourth seal judgment (Revelation 6:8), and now an equal number are killed by the Euphrates demons. All men will realize by this time that they are suffering because of their rebellion against their Creator and the One who came as their proffered Redeemer, but most will still (perhaps with a few exceptions) refuse to repent (Revelation 9:20).