The Government Shall Be Upon His Shoulders

In order to forestall tyranny, the United States adopted a "separation of powers" wherein in no branch of government is supposed to wield ultimate authority.  Yet, in the Nicene Creed, Christians proudly proclaim that when Christ comes again "His kingdom shall have no end."  How does God combine the powers necessary for good government in the Person of Christ?  The gifts of the Magi point the way for us...