The Fourth Horseman is the fascinating if at times necessarily speculative account of the son of an American hero who plotted to use chemical and biological weapons (anthrax, for one) to aid the Germans in World War I and attempted to push Mexico into an invasion of the United States.

In a story that has been shrouded in mystery until now, Robert Koenig details (at least as much as possible) the nebulous career of a spy who was so adept at his sickening work that it is probable that even the unscrupulous regime to which he gave his allegiance, the German High Command, were responsible for his untimely and as yet still unconfirmed murder.

The Fourth Horseman reads like a fast-paced novel; at times it is hard to remember that this was a flesh-and-blood man who betrayed his country for a reason that still remains unknown. With meticulous research, Koenig brings to life a man who is little-known to the public but who is a legend in the annals of the underworld of spies.