I saw the beast
This is the Beast who rose from the abyss in a miraculous restoration from death and was worshiped by the earth dwellers. See The Beast. See #16 - Beast. See commentary on Revelation 13:1. The angel previously told John, These [the Beast and the ten kings] will make war with the Lamb, and the Lamb will overcome them (Rev. Rev. 17:14+).
the kings of the earth
Beyond the ten kings allied with the Beast, Scripture mentions the kings from the east as well as the kings of the earth (Rev. Rev. 16:12+, Rev. 16:14+). The Beast and his ten kings destroyed Babylon (Rev. Rev. 17:16-18+) while the rest of the kings of the earth lamented her destruction (Rev. Rev. 18:9+).
It is these specific kings of history, the kings of the earth at the time of the end, which provide the ultimate fulfillment of Psalm Ps. 2:1: The kings of the earth set themselves and the rulers take counsel together against the LORD and against His Anointed [Messiah = Christ], saying, Let us break their bonds in pieces and cast away their cords from us (Ps. Ps. 2:2-3). These are the final kings of history who fail to kiss the Son, lest He be angry so they perish in the way (Ps. Ps. 2:12).
gathered together to make war against Him
Gathered together is συνηγμένα [synēgmena] , perfect tense: having already gathered together. This is the gathering which was initiated by the three unclean spirits like frogs which go out to the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty (Rev. Rev. 16:14+). Having unknowingly followed Joels instructions, they have beat their plowshares into swords and their pruning hooks into spears (Joel Joel 3:9). This is Gods assembly of kingdoms upon which He promised to pour out His indignation and fierce anger (Zep. Zep. 3:8). See Campaign of Armageddon.
We may wonder how rational men could be carried with one impulse into an attempt so daring and so absurd; but when people put the truth from them, and submit themselves to the Devils lead, what is there of delusion and absurdity into which they are not liable to be carried?1
1 J. A. Seiss, The Apocalypse: Lectures on the Book of Revelation (Grand Rapids, MI: Zondervan Publishing House, 1966), 440.