Shortly after the U.S. 3rd Army liberated the Buchenwald concentration camp in April 1945, a 27-year-old intelligence officer named Albert Rosenbergbegan gathering evidence of the atrocities committed there, horrors that would become a keystone of coming war crimes trials and the voluminous literature documenting Nazi wickedness. Lieutenant Rosenberg was no ordinary GI. He was a Jew born in Germany who, in 1937, had left his home country for the United States after becoming the target of Nazi violence. He’d been drafted into the Army in 1942, becoming one of the 2,000 or so young Jewish German exiles deployed as interrogators and spies against their erstwhile countrymen.

Rosenberg, a native German speaker who was also fluent in English and French, made a powerful impression on French resistance fighter Jorge Semprún, one of the Buchenwald survivors who helped interpret the camp’s infernal workings. Rosenberg was a “thin, gangly intellectual with a sad and piercing gaze,” Semprún would recall years later. “He had become an American to bear arms, to make war on Nazism. To make war on his own country. By becoming an American, he had chosen the universality of the democratic cause, an abstraction that could not become reality until his country had been defeated.”

In late 1942, the Army decided to create a special program to make use of the émigrés’ familiarity with Germany and its language—bright and bilingual, and highly motivated, they were well equipped to extract secrets from German POWs. After being plucked from other units, the men were hastily naturalized, trained in interrogation and propaganda techniques, shipped off to England and attached in small units to the combat forces that invaded Normandy and subsequently defeated and occupied their former homeland. They provided roughly two-thirds of the human intelligence used in the fight against Nazi Germany, according to an official estimate quoted in Sons and Soldiers, military historian Bruce Henderson’s sparkling new account of these émigré-fighters.

These newly hatched Americans trained mostly at Fort Ritchie, a mountain base near Hagerstown, Maryland, that was unconventional in every way. “The soldiers, moving about with a thoroughly unmartial gait, conversing in languages never heard in the Appalachians, might have been taken for Central European vacationers at a spa, gossiping about their cures,” Henderson quotes one émigré as saying. Though the recruits learned how to use rifles and march and drill, and occasionally took long forays into the woods—they frequently got lost and were rescued by puzzled farmers—the bulk of their eight-week training courses consisted of learning German military structures and other information crucial for extracting secrets from captive Wehrmacht soldiers.

The “Ritchie Boys,” as they would become known, employed guile and smarts to glean insights from the POWs while respecting the Geneva Accords. They learned how to distinguish between soldiers with greater or lesser adherence to Nazi ideals; to use bits of intelligence picked up from one soldier to present themselves to a different detainee as all-seeing, making the soldiers feel hopeless about withholding information. One tactic was to present unhelpful POWs with the false impression that they would be shipped to much-feared Russian POW camps if they failed to cooperate. But notably, the émigré soldiers never used waterboarding, electric shock or other “enhanced techniques.” In itself, this was a remarkable fact given the horrible casualties they saw inflicted on their comrades daily, as well as their own deeply personal motivations for seeking revenge on the Germans, who had persecuted and murdered their families and forced them into exile in the first place. They fervently believed that stooping to torture would render them no better than the Nazis. ...