Planet of the Damned (Unabridged‪)‬

Two planets are about to engage in a nuclear war that will destroy one of them. Millions will perish. Brion Brandd, who lives on neither planet, has been recruited by the Cultural Relationships Foundation to stop the war from happening. He is the only person in the galaxy who can do it, but can he? 

A classic science fiction short novel by master Harry Harrison.