The circle of “kings of the whole world” is obviously bigger than the circle of kings from the east.
The point of the drying of the Euphrates was that (natural) protection from foreign invasion was removed so as to upset a nation’s sense of security. The Lord does not say that only kings or nations situated to the east could possibly be a threat.
14 For they are demonic spirits, performing signs, who go abroad to the kings of the whole world, to assemble them for battle on the great day of God the Almighty.