City of Endless Night

It is quite difficult to define if this book belongs to fiction or non-fiction. City of Endless Nigh describes the author's vision of the future after World War I if it had ended in a different way. In this stories Germans do not surrender and just go underground and build a city there where they are preparing for a revanche. The allied forces cannot understand where the rebels are to destroy them. A citizen of the U.S. somehow gets to this underground city and sees all the terrible scenes there