The devil is the ruler of this world, but with the Lord's OK as to who will rule.
The Lord allows certain individual's whom He deems appropriate to rule over different peoples at different times.
The Lord allowed Nebuchadnezzar and the Chaldeans to conquer unjust Israel, and other unjust Middle Eastern nations, as a lesser evil than having those nations continue in their escalating unjust behavior.
The Lord allowed the Conquistadors to conquer and enslave Central and South America as a lesser evil than letting the barbaric Aztec and Inca Empires continue exterminating the smaller tribes of indigenous people with whom they coexisted.
As Satan's world continues to disintegrate, the devil will soon be cast to the earth, where he will then have one last go at making things work. (Revelation 12:9 & 12).
From the current disintegration of policies throughout the world, I think it may be a short matter of time before the devils last stand.
Revelation 12:9 And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.
12 Therefore rejoice, ye heavens, and ye that dwell in them. Woe to the inhabiters of the earth and of the sea! for the devil is come down unto you, having great wrath, because he knoweth that he hath but a short time.