Jamieson, Fausset and Brown on Daniel 2:37

37. Thou ... art a king of kings--The committal of power in fullest plenitude belongs to Nebuchadnezzar personally, as having made Babylon the mighty empire it was. In twenty-three years after him the empire was ended: with him its greatness is identified (Da 4:30), his successors having done nothing notable. Not that he actually ruled every part of the globe, but that God granted him illimitable dominion in whatever direction his ambition led him, Egypt, Nineveh, Arabia, Syria, Tyre, and its Phoenician colonies (Jr 27:5-8). Compare as to Cyrus, Ezr 1:2.