Book Review: The City and the Stars by Arthur C. Clarke

Once upon a time man crossed the stars and spread throughout the galaxy, leaving Earth behind.  They met other intelligent life forms and formed alliances.  Unfortunately, this took place a billion years ago.  In the interim they ran across a species that was not happy about man's success, known only as The Invaders, and by launching war they forced humanity to abandon the galaxy and retreat to Earth.  During the war that resulted large portions of the earth were subject to desertification and the surviving inhabitants were forced to flee to a lone city, Diaspar.