In 2007, under the watch of insolently lounging house cats in a Northern California living room, I opened the lids of four battered cardboard boxes. Out rushed the smell of a vanished world: a physician’s patient records and hand-written notes untouched for decades, disintegrating photographs, stale cigarette smoke, x-ray images of Adolf Hitler’s skull, wax-sealed packets of narcotics, and allegedly poisoned food. The boxes exhaled the air of Nuremberg, Germany, from 1945 to 1946, when 22 of the top Nazi leaders awaited and began their trial by an International Tribunal. The author of the materials in these boxes, US Army psychiatrist Douglas M. Kelley, was the first mental health expert to examine the Nazi prisoners.

I arrived to look at the boxes at the invitation of Kelley’s son. In August 1945, his father had received orders to go to a hotel in Luxembourg converted into a temporary prison, where Kelley met such infamous Nazi leaders as Hermann Göring, Joachim von Ribbentrop, Julius Streicher, Ernst Kaltenbrunner, and Karl Dönitz. Dr. Kelley’s assignment was to certify their mental competence for the trial to come. Just 33 and supremely confident in his abilities, the psychiatrist had more ambitious plans for his time spent with the men widely regarded as the worst criminals of the twentieth century. He wanted to discover the essence of the “Nazi personality.” If he could identify mental disorders and psychological traits common to the Nazi leaders, he could point to others among us capable of committing similar crimes....