Jamieson, Fausset and Brown on Daniel 4:12

12. beasts ... shadow under it--implying that God's purpose in establishing empires in the world is that they may be as trees affording men "fruits" for "meat," and a "shadow" for "rest" (compare La 4:20). But the world powers abuse their trust for self; therefore Messiah comes to plant the tree of His gospel kingdom, which alone shall realize God's purpose (Eze 17:23; Mt 13:32). Herodotus [7.19] mentions a dream (probably suggested by the tradition of this dream of Nebuchadnezzar in Daniel) which Xerxes had; namely, that he was crowned with olive, and that the branches of the olive filled the whole earth, but that afterwards the crown vanished from his head: signifying his universal dominion soon to come to an end.