The Millennial, 1000 year, reign of Jesus Christ will be a Theocracy.  God will rule by Jesus Christ.
Luke 1:30-33


"And the angel said unto her, Fear not, Mary: for thou hast found favour with God.
And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name Jesus.
He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David:
And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end."