Robert Koenig's meticulously researched book The Fourth Horseman is a rattling good read. The tale speeds along like a spy thriller -- which it is -- but it is also packed with esoteric facts and entertaining details on subjects running the gamut from the pivotal role of warhorses before the advent of the jeep to the propagandist voyage to the United States of the German U-boat Deutschland.The central figure is Anton Dilger, MD, alias Dr. Delmar, the handsome son of German immigrants to the U.S. Born in Virginia, Dilger received his medical education in Germany and was recruited there as a spy and germ saboteur. He met his demise in Spain at the war's end under mysterious circumstances.Even before the U.S. entered World War I, it supplied hundreds of thousands of horses and mules to Europe, as both cavalry and pack animals, to support the Allied war effort. The...

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