The Fourth Horseman

One Man's Mission to Wage the Great War in America

By Robert Koenig

PUBLICAFFAIRS; 349 Pages; $26

In 1916, Anton Dilger, an American saboteur working for the German government, rented lodgings not 6 miles from the White House. In his basement, he set up a small laboratory and, on behalf of the General Staff in Berlin, he began a highly secret campaign to wage biological warfare on U.S. soil. His target would be the horses and cattle supplied to the Allied armies by the then-neutral United States, and Dilger set about cultivating anthrax bacteria and Pseudomonas mallei, the germ that causes glanders, a crippling equine disease. But who was Dilger, and how did the son of a Union Army captain become a German secret agent?

Author Robert Koenig has pieced together a detailed portrait using family letters, postcards, archives and oral history. Dilger's father, Hubert "Leatherbreeches" Dilger, was a hero of Chancellorsville and Gettysburg, and a recipient of the Medal of Honor. Settling in Virginia's lush Shenandoah Valley after the Civil War, the German immigrant purchased 1,800 acres near the Blue Ridge Mountains; it was among these rolling hills that Anton Dilger was born in 1884, and where he spent an idyllic childhood riding the horses raised by his father.

But Anton's mother wanted something "more than horses" for the boy, and when Anton's sister Eda married a Mannheim businessman, the 10-year-old boy accompanied them to Germany. Buoyed by his brother-in-law's wealth, Anton worked hard at his studies, eventually winning a position at the University of Heidelberg and studying medicine.

At 18, Anton returned to the United States for a visit, his first in eight years. But everything was different: Rural Virginia seemed primitive compared with the cities of Germany, and his parents had aged considerably. After returning to Germany, Anton quickly completed his medical studies, and he accepted a position as an assistant surgeon. Determined to be a professor of surgery, Anton found his plans derailed when the First Balkan War broke out in 1912. The young doctor was asked, by Queen Eleonore herself -- the Bulgarian tsaritsa and a native German -- to become a field surgeon. But the experience benefited Dilger: Battlefield service burnished his credentials, gave him greater confidence and whetted his interest in military intrigue.

When the Great War began, Dilger was once again in Heidelberg. The war was just 2 weeks old when he received word that his nephew, who was fighting for the Germans, had been killed by a French sniper. Anton debated what to do: Should he return to America and remain neutral, or should he enter the conflict? As an American citizen, Anton was excluded from German military service, but he was able to volunteer as a noncombatant surgeon near the Western Front. He chose to stay.

By 1914, the cavalry accounted for almost a third of most of Europe's armies, and was regarded as the key element in offensive battles. All that was about to change, as mounted soldiers faced machine guns for the first time. Cavalry regiments were massacred, and the once-fluid battlefield transformed into fixed trench warfare. While Anton treated horrific battlefield injuries, he questioned the neutral stance of the United States, which allowed for shipments of munitions, food and horses to the Allied armies. Frustrated by his behind-the-lines work as a physician, Anton pushed for a more active role in the war.

In 1915, the first year that the German army deployed poison gas on the battlefield, a unit of the General Staff in Berlin began a campaign to use germs as weapons of sabotage. The covert operation required someone with medical expertise, who could enter the United States without arousing suspicion -- and who was utterly loyal to Germany. That person was Anton Dilger.

Although the cavalry was relegated to a secondary role during the war's remaining years, horses and mules were still needed to pull supply wagons, haul large guns and support reconnaissance missions. It was these animals that Dilger sought to eliminate. In his basement laboratory, he tested his concoctions on a cage of guinea pigs; when the animals died, he was ready for the next step. After placing the cultures into more than 30 bottles, Dilger showed his fellow saboteurs how to disseminate the bacteria: Anthrax should be injected into the horses, while the glanders germs could be rubbed inside the horses' nostrils or poured into their feed or water troughs. Agents headed for ports in Baltimore and Newport News and Norfolk, Va., infecting transports all winter. It's difficult to determine how many animals perished; diagnostic tests for glanders did exist, which helped blunt the impact of the sabotage.

As anti-German sentiment increased, the saboteurs moved their lab to the Midwest. Reports at the time noted the emergence of severe outbreaks of disease among horses and mules. Army veterinarians noted that disease also struck the American horses being shipped to the British and French armies. But the focus of Dilger's espionage work was about to change.

In an infamous secret message intercepted by the British, Germany proposed an alliance with Mexico that would allow the Mexicans to "reconquer its lost territory in Texas, New Mexico and Arizona," backed by generous financial support from Berlin. When the United States entered World War I shortly thereafter, Dilger secretly obtained German citizenship, and he traveled to Mexico City. His mission: to bribe or goad Mexico into invading the United States.

In this final section of the book, Koenig uncovers the dissension among German intelligence agents and how the Allied secret services identified these men. As his mission unraveled, Dilger left Mexico City for Madrid. There, the young doctor, just 34, contracted Spanish flu. He died Oct. 17, 1918, the germ saboteur felled by a virus.