The vision inserted between the sixth and seventh bowls is the gathering for the war at Armageddon (Rev. 16:13-16).
The gathering at Armageddon takes place by the three unclean spirits out of the mouths of Satan, Antichrist, and the false prophet, the satanic trinity, going forth to stir up the kings of the whole inhabited earth (Rev. 16:13-14). The three unclean spirits go out to stir up these kings before the sixth bowl. The sixth bowl will dry up the Euphrates to prepare the way of the kings. These kings have to travel to reach the Euphrates, so the three unclean spirits stir them up to come.
Antichrist and his armies will fight against Christ and His army (Rev. 16:16; 17:12-14; 19:11-19). Christ and His chosen overcomers, His army, will defeat them. This will be the destruction of the destroyers of the earth.
The gathering of the Antichrist, the false prophet, and the ten kings, with their armies is the collecting of the "grapes" into the winepress. The war at Armageddon is the treading of the great winepress of the fury of God (Rev. 14:17-20; Isa. 63:1-6).
Revelation 16:15 speaks of the Lord's reminding of His remaining believers concerning His coming as a thief. This warning should be before the seventh trumpet (1 Cor. 15:52).
The pouring out of the seventh bowl brings in the greatest earthquake ever and the great hail of the exceedingly great plague (Rev. 16:17-21).