Jesus never said there would be a literal kingdom on earth.
And yet it was what the Jews were expecting.....even as Jesus was ascending to heaven, they asked him....
"Lord, are you restoring the kingdom to Israel at this time?” (Acts of the Apostles1:6) It was not until Pentecost that the anointing of his 120 disciples revealed the truth....the kingdom of God was in heaven, and they were going there to rule with their Lord Jesus. Holy spirit convicted them of that hope, which was all consuming.
As time went on, the truth, (the sacred secret) hidden from the founding of the world, was progressively revealed through the apostles after Jesus' death. (Romans 6:25; Ephesians 1:4) God gave his first prophesy in Eden, before Adam and his wife had children, (Genesis 3:15) but no one understood the details of it until the holy spirit began to become operative in the first century. The "light on the path" (Proverbs 4:18) continues to get brighter as the day for God's judgment (with Jesus and his angelic forces at the helm,) brings this current world system under the control of the devil, to its finish.
What end? The end of an era and a new beginning.
Man's rulership under influence from the devil will cease. (1 John 5:19) God will bring his rulership back to the earth with Jesus as king, reconciling redeemed mankind with their Creator, regaining the paradise conditions on this earth that Adam forfeited. (Revelation 21:2-4)
This is why Jesus said that, as part of the "sign" that he gave his apostles to indicate that his Kingdom had begun ruling, a world wide witness had to be given before the "end" would come. (Matthew 24:14)
Jesus kingdom was not of the Roman Empire.
Jesus did not interfere with the Roman empire because the "appointed times of the nations" had to run its full course.
He said...
"Jerusalem will be trampled on by the nations until the appointed times of the nations are fulfilled." (Luke 21:24)
The Gentile nations that would dominate God's people had been foretold by Daniel. Beginning with Babylon, then Medo-Persia, then Greece, then Rome, and out of the ashes of Rome came Britannia, to be joined in the time of the end by America. This is the last ruling power before the coming of God's kingdom which will crush all failed human rulership out of existence, and replace them. (Daniel 2:44)
The kingdom of God is not something which will one day come, for it already exists. Jesus said:
“if I cast out demons by the Spirit of God, then the kingdom of God has come upon you” (Mt. 12:28).
“The kingdom of God is not coming with signs to be observed; nor will they say, ‘Look, here it is!’ or, ‘There it is!’ For behold, the kingdom of God is in your midst” (Luke 17:20-21).
The Kingdom is what Jesus taught us to pray for. Daniel tells us plainly that it is a real government of God which will "come" and take over control of this earth after every last vestige of this wicked old system has been removed. (2 Peter 3:11-13)
This is not a new planet, but one cleansed of all wickedness and those who practice it. Those who have ruined his earth will pay the penalty according to Revelation 11:18....
"But the nations became wrathful, and your own wrath came, and the appointed time came for the dead to be judged and to reward your slaves the prophets and the holy ones and those fearing your name, the small and the great, and to bring to ruin those ruining the earth."
When that was written, man did not have the capacity to ruin the earth....now he has the ability to wipe out all life. The wrath of man will be no match for the wrath of God.
The Kingdom was represented by Jesus when he walked the earth. When Jesus said that "
the kingdom of God is in your midst" he was speaking to the Pharisees who had already determined that this man would never be King over them....but as the appointed King of God's kingdom, there he was walking right among them and they failed to recognize him.