Norman Davies, Vanished Kingdoms: The History of Half-Forgotten Europe (Allen Lane, 2011), 830 pages, $59.95 Crumbled empires, empty thrones and vanquished powers. Each is an endless source of human fascination. It’s no surprise that the greatest work on Rome tells the story of its decline and fall. Norman Davies’s new book, Vanished Kingdoms, is the latest contribution to the field. A distinguished Oxford historian, Davies asks: Why do states vanish, and what does it mean when they do? Davies is aware of the ancient genesis of these questions. The monument to Ozymandias pokes out of the sand just nine pages…