The Euphrates River flowed from north to south some distance east of the Promised Land. The river was big enough to form a natural barrier that repeatedly made it very difficult for armies from the east to march west and conquer western lands. By pouring his bowl on this river, the sixth angel removed this barrier and so opened the way for “the kings from the east” to invade westward. In John’s day the Parthians dominated the political landscape east of the Euphrates; their kingdom extended from time to time to include portions of present-day India and even China. Drying the river meant that the peace and security of the Roman Empire was threatened. Obviously, that threat would affect also the regions of Asia Minor where the seven churches were located (to whom John had to send the seven letters and the rest of the revelation he received; see Revelation 1:11).
12 The sixth angel poured out his bowl on the great river Euphrates, and its water was dried up, to prepare the way for the kings from the east.