The kings of the east are found in the following verse:
"And the sixth angel poured out his vial upon the great river Euphrates; and the water thereof was dried up, that the way of the kings of the east might be prepared." Revelation 16:12
While the drying up of the great river Euphrates prepares the way for the kings of the east, these kings do not actually cross over the river yet.
Consider another occurrence of the word "east" in the book of Revelation: "And I saw another angel ascending from the east, having the seal of the living God: and he cried with a loud voice to the four angels, to whom it was given to hurt the earth and the sea" Revelation 7:2
The angel ascending from the east have finished sealing the 144,000 livings saints upon the earth by this point during the sixth plague.
The kings of the east are prepared to battle, their way is prepared, but they are not engaging in the conflict yet.
What are the kings of the east waiting for?
"And I saw three unclean spirits like frogs come out of the mouth of the dragon, and out of the mouth of the beast, and out of the mouth of the false prophet. For they are the spirits of devils, working miracles, which go forth unto the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty." Revelation 16:12
Satan is at the same time preparing the way for the kings of the earth and of the whole world to fight against the kings of the east and their allies, which is the army of God Almighty..
While the kings of the earth at this point DO NOT see the invisible army of God, they DO see the kings of the east having been sealed by the angel ascending from the east.
Before the drying up of the great river Euphrates, the world is engaged in a world wide war. But after the drying up of the great river Euphrates, the world's attention changed, and it is about to engage in a different kind of war: battle against God's covenant people, the 144,000.
Why are the covenant people, 144,000 called the kings of the east?
"For the great day of his wrath is come; and who shall be able to stand?" Revelation 6:17
"Who raised up the righteous man from the east, called him to his foot, gave the nations before him, and made him rule over kings? he gave them as the dust to his sword, and as driven stubble to his bow." Isaiah 41:2
The righteous from the east shall STAND
"And hath made us kings and priests unto God and his Father; to him be glory and dominion for ever and ever. Amen." Revelation 1:6
We are the kings of the east, and our way is JESUS, and when the great river Euphrates is dried up, and the world turns its attention to us, then shall we hear God's covenant and shall lift up our eyes, and see Jesus coming in the clouds of glory.