Jamieson, Fausset and Brown on Revelation 9:14

14. in, &c.--Greek, "epi to potamo"; "on," or "at the great river."

Euphrates--(Compare Re 16:12). The river whereat Babylon, the ancient foe of God's people was situated. Again, whether from the literal region of the Euphrates, or from the spiritual Babylon (the apostate Church, especially Rome), four angelic ministers of God's judgments shall go forth, assembling an army of horsemen throughout the four quarters of the earth, to slay a third of men, the brunt of the visitation shall be on Palestine.