RUTHFULLY YOURS

By STEPHEN MILLER

Shortly after German troops invaded Belgium in 1940, Gaston Vandermeerssche, a Belgian university student, bicycled 800 miles to the south of France and became a spy.

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Vandermeerssche’s familyGaston Vandermeerssche at the electron microscope in Ghent, Belgium.

Mr. Vandermeerssche, who died Nov. 1 at age 89 in Milwaukee, joined the resistance and ferried microfilm documents over the Pyrenees to Spain, where intermediaries sent the information on to London.

Later in the war he helped organize the Dutch underground, which came to comprise hundreds of agents and safe houses. After his network was penetrated by the Germans, he tried to escape, but was arrested near the Spanish border. He spent 24 months being interrogated in prison, but by his own account never broke.

His wartime exploits are in few history books, but espionage experts and Belgian state archives corroborate his account.

Born in Ghent, Mr. Vandermeerssche was the son of a furniture maker. He was a student at Ghent University when war broke out, and immediately joined the Belgian army. When the Germans occupied Belgium, he worked for the resistance, distributing an underground newspaper.

Frustrated and in danger, he fled Belgium via bicycle and joined the resistance in Toulouse, France. He became a courier, making weekly trips from Brussels to Toulouse to Barcelona. The last leg involved trudging over snowy passes in the Pyrenees by moonlight. The microfilms he carried bore information collected by members of the underground on shipyards, gun emplacements and the like. On the return trip, he carried money to fund the resistance.

Mr. Vandermeerssche began setting up safe houses and clandestine mail drops, delving deeper into espionage. His contacts included Britain’s MI6, which dropped him by parachute on a mission to organize the Dutch underground, according to Nigel West, a World War II espionage authority.

Mr. Vandermeerssche told an interviewer that he found a nascent underground in place in the Netherlands.

“They had groups—twenty, thirty, forty people organized—and they couldn’t send the messages. They had no radio. At that time, the British were using pigeons,” Mr. Vandermeerssche said in the 1999 oral history “A Time To Speak.”

But much of the Dutch underground was compromised by German infiltrators, to the point where dozens of intelligence agents were apprehended while parachuting into the Netherlands.

Mr. Vandermeerssche was arrested in Perpignan, France, in 1943 with a cache of microfilm stuffed into butter tubs. His German interrogators suspected his role in the Dutch underground, but couldn’t prove it.

“I was so young, the Germans did not believe that this kid was the head of that large network,” he said in the oral history. “And I told them, ‘Are you crazy? I couldn’t have done this.’ ”

Months of brutal interrogation and solitary confinement failed to break Mr. Vandermeerssche’s will. He was betrayed by another member of the underground, and was sentenced to death in a military trial. But he was freed by American troops near the end of the war.

Although shattered by his experiences in prison—he said he couldn’t eat or sleep normally for a decade—Mr. Vandermeerssche resumed his studies, earning a Ph.D. in physics. He ran Ghent University’s electron-microscope department. In 1965, he moved to the U.S. and later became an executive at the Joseph Schlitz Brewing Co. He specialized in tribology, the physics of surface abrasion, a topic that interested brewers whose can labels were damaged during shipping.

Mr. Vandermeerssche seldom spoke about his wartime experiences, even to his family. He was decorated by five countries and in 1980 was named honorary French consul for Milwaukee.

In the 1980s, he began visiting Europe to reconstruct his wartime activities, and recounted them in a 1988 book, “Gaston’s War.” He came to believe that his spy networks had been purposely exposed by his masters in London, as a diversion to convince the Germans that D-Day invasions were planned for the Low Countries instead of Normandy. He called it “le grand jeu”—the great game—in his memoir.

“Now I’m not bitter at all,” he said in the oral history. “It was an honor that I was used for the big purpose.”

—John Miller contributed to this article. —Email remembrances@wsj.com