Tourists who wander Cambodia’s Killing Fields don’t just encounter the ghosts of victims. Even today, scraps of clothing and bone fragments belonging to some of the 1.7 million people slaughtered by the Khmer Rouge peek through the brooding earth.

From 1975 until the Vietnamese invaded in 1979, Cambodia experienced one of the worst genocides of the 20th century, during which around a quarter of the population perished as Pol Pot pursued his demented agrarian utopia.

Phnom Penh’s glorious Parisian-style boulevards were emptied as ruthless cadres — many mere babes handed AK-47s and indoctrinated with nihilistic zeal — forced the entire population to toil in the fields and ruthlessly culled anyone on the flimsiest pretense. Crying at a funeral, falling ill or wearing eyeglasses were deemed anti-revolutionary and met with torture and gruesome death.

“For 20 years, I had nightmares every day, and every time I talked about what happened I would get stomachaches and all the symptoms of PTSD,” says Kilong Ung, who was 15 when the Khmer Rouge turned his life upside down and extinguished the lives of 50 of his relatives....