"They are also seven kings. Five have fallen, one is, the other has not yet come; but when he does come, he must remain for only a little while. The beast who once was, and now is not, is an eighth king. He belongs to the seven and is going to his destruction." (Revelation 17: 10-11)
I think “seven kings” may refer to seven particular governments of humanity since the creation of Adam beginning with Babel. Six have come and gone - Babel, Egypt, Babylonia, Persia, Greece, and Rome. Five of those Empires had fallen by the
time John wrote the book of Revelation and the sixth was the Roman Empire in power at the time.
The seventh government may have been the British Empire at its zenith. At its height, the
British Empire, a government "Empire on which the sun never sets". It gained administration of the promised land after World War One and in 1917 Britain publicly pledged to establish a national home for the Jewish people in Palestine. The Jews declared themselves the independent State of Israel on March 14, 1948.
The United States of America seems to fit as the eighth Empire. It came out of the British Empire, and its citizens are children of immigrants from
all over the world. So, the USA may be modern day Babel, the kingdom who "once was, is now is not, and is the eighth".
"Now we ask you, brothers and sisters, regarding the coming of our Lord Jesus Christ and our gathering together to Him, that you not be quickly shaken from your composure or be disturbed either by a spirit, or a message, or a letter as if from us, to the effect that the DAY OF THE LORD has come. No one is to deceive you in any way! For it will not come unless the apostasy comes first, and THE MAN OF LAWLESSNESS IS REVEALED, the son of destruction, who opposes and exalts himself above everyone called god or object of worship, so that he takes his seat in the temple of God, displaying himself as being God. Do you not remember that while I was still with you, I was telling you these things? And you know what RESTRAINS HIM now, so that HE WILL BE REVEALED IN HIS TIME. For the mystery of lawlessness is already at work; only He who now restrains will do so until he is removed. Then that lawless one will be revealed, whom the Lord will eliminate with the breath of His mouth and bring to an end by the appearance of His coming." (2 Thessalonians 2:1-8)
The "day of the Lord" refers to a period of time just before Jesus returns - the 70th seven of Daniel's prophecy concerning Israel. The "apostasy" is a world-wide falling away from the gospel of God which began in Paul's time and is rampant today. All that remains is for the power of God retraining the wrath of the last days to be removed.