Philip K. Dick 
Second Variety [PDF ebook] 

Influential American science fiction writer Philip K. Dick was fascinated by the idea of nuclear warfare and the dire aftermath of such an event. This scenario preoccupied Dick throughout much of his career and served as the backdrop to many of his stories and novels. In the short story "Second Variety, " the human population has been decimated by nuclear war, and small tribes of survivors are forced to battle a species of lethal robot soldiers.