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More than 80 years before envelopes filled with white powder sent the country into a panic, America had another encounter with bioweapons. As journalist Robert Koenig chronicles in The Fourth Horseman: One Man’s Secret Campaign to Fight the Great War in America, one Anton Dilger—a German-American doctor raised on a Virginia horse farm—used his medical skill and equine expertise to sabotage the American military’s horse-breeding efforts during World War I with anthrax and other deadly germs. Koenig, a journalist with a long history covering German affairs, traces Dilger’s path to infamy from his ironic roots—Dilger’s father was a decorated Union cavalier in the Civil War—to the rented house on 33rd Street NW in Chevy Chase where, as a secret agent of the Kaiser, he cultivated the pathogens he would eventually use to sabotage American war horses. More than that, Koenig explains the big picture: the historical importance of the war horse, the deeply conflicted feelings of German-Americans during the First World War, and the strategy and tactics of early-20th-century skulduggery. And, like just about every book of popular history released these days, The Fourth Horseman holds a contemporary lesson: A smart, socially adept, smooth-talking professional such as Dilger is just the sort of foreign operative that could do great harm to national security today. Koenig discusses and signs copies of his book at 7 p.m. at Politics and Prose, 5015 Connecticut Ave. NW. Free. (202) 364-1919.