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. (Revelation 16:12)
When the sixth angel pours out his bowl over the whole world the great river Euphrates will dry up. This will allow the great armies of Asia to cross over towards the Middle East. This is probably intended to allow them to be present (and be destroyed) at the great battle of Armeggedon with the rest of the world’s armies (Revelation 16:16, 19:17-21). It is also possible that they will contribute to the destruction of Babylon along the way (Revelation 17:16, 18:2, Isaiah 13:6,17).
There are several Old Testament passages that also predict that the Euphrates River will be dried up. It appears from these verses that God will make a highway through it (and the Nile, which will similarly be destroyed) that will allow the remaining Jews that have been scattered all over the world to return to Jerusalem.
The Lord will dry up
the gulf of the Egyptian sea;
with a scorching wind he will sweep his hand
over the Euphrates River.
He will break it up into seven streams
so that anyone can cross over in sandals.
16 There will be a highway for the remnant of his people
that is left from Assyria,
as there was for Israel
when they came up from Egypt. (Isaiah 11:15-16)
In that day the Lord will thresh from the flowing Euphrates to the Wadi of Egypt, and you, Israel, will be gathered up one by one. 13 And in that day a great trumpet will sound. Those who were perishing in Assyria and those who were exiled in Egypt will come and worship the Lord on the holy mountain in Jerusalem. (Isaiah 27:12-13)
The waters of the river will dry up,
and the riverbed will be parched and dry.
6 The canals will stink;
the streams of Egypt will dwindle and dry up.
The reeds and rushes will wither,
7 also the plants along the Nile,
at the mouth of the river.
Every sown field along the Nile
will become parched, will blow away and be no more.
8 The fishermen will groan and lament,
all who cast hooks into the Nile;
those who throw nets on the water
will pine away.
9 Those who work with combed flax will despair,
the weavers of fine linen will lose hope.
10 The workers in cloth will be dejected,
and all the wage earners will be sick at heart. (Isaiah 19:5-10)