Who gathers the kings of the earth to battle?
The kings of the earth are gathered to the battle of Armageddon by three unclean spirits, like frogs, that come out of the mouths of the dragon, the beast, and the false prophet, working miracles to deceive and unite them for war.
Scripture shows that it is not Christ who assembles this final army, but the spirits of devils sent out by the unholy trinity of the dragon, the beast, and the false prophet. Revelation 16 describes three unclean spirits like frogs coming out of their mouths, and these spirits go forth unto the kings of the earth and of the whole world to gather them together for the battle of that great day of God Almighty. The place of this gathering is called, in the Hebrew tongue, Armageddon.
This gathering happens after the sixth vial is poured out upon the river Euphrates, drying up its waters to prepare the way for the kings of the east to come. So the kings of the earth are deceived and drawn together by demonic miracle working spirits acting on behalf of the beast and the false prophet, all under the ultimate direction of the dragon, Satan himself.
Once assembled, these kings and their armies are said to gather together to make war against Him that sat on the white horse, meaning Christ and His heavenly army. Their gathering for battle against Jerusalem is also described as the Lord Himself gathering all nations against the city, showing that even though the beast's spirits stir them to war, it is ultimately within the sovereign purpose of the Lord that they are brought to that place of judgment, the valley of decision, where His wrath is poured out upon them.
The scriptures
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. 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. Behold, I come as a thief. Blessed is he that watcheth, and keepeth his garments, lest he walk naked, and they see his shame. And he gathered them together into a place called in the Hebrew tongue Armageddon.
And I saw the beast, and the kings of the earth, and their armies, gathered together to make war against him that sat on the horse, and against his army.
For I will gather all nations against Jerusalem to battle; and the city shall be taken, and the houses rifled, and the women ravished; and half of the city shall go forth into captivity, and the residue of the people shall not be cut off from the city.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.