Of all the modern genocides, the mass slaughter of two million Cambodians under the Khmer Rouge between 1975 and 1979 was probably the least understood at the time of the killing. Cambodia was cut off from the world. With the war in Vietnam over, the United States and other great powers stopped paying attention to Indochina, as did most of the international press. After the Vietnamese Army overthrew the Khmer Rouge, in January, 1979, the United Nations continued to recognize the deposed regime as the legitimate government of Cambodia. The Vietnamese liberators-turned-occupiers lacked credibility on the subject of human-rights abuses, although they had put an end to the worst of them in Cambodia. Cold War thinking led Western and Southeast Asian countries and China to back the Khmer Rouge for years.

Reports by survivors came out in bits and pieces, almost all of them after the killing was over. The survivors had few international advocates, and many people around the world, especially on the left, didn’t believe them. It took a Hollywood movie, “The Killing Fields,” to bring the destruction of a quarter of Cambodia’s population to the world’s attention. The Cambodian genocide was an overlooked historical moment between the epic horror of the Holocaust and the more recent genocides in Rwanda and Bosnia that took place under the world’s gaze.

Thirty years after the fall of the Khmer Rouge, in 2009, a joint Cambodian-U.N. tribunal in Phnom Penh began to hear cases against the handful of alleged perpetrators who are still living. The first defendant was Kaing Guek Eav, whose revolutionary name was Duch (pronounced Doik). Duch was the director of S-21, the Khmer Rouge’s central interrogation center and death camp, from which very few people emerged alive. “The Master of Confessions: The Making of a Khmer Rouge Torturer,” by the French journalist Thierry Cruvellier, has just been published in translation here in the United States. Cruvellier (who is a friend of mine) has covered all the contemporary international courts for war crimes and crimes against humanity, spending the past fifteen years immersed in the details of the genocides in Rwanda and Cambodia, the ethnic cleansing in Bosnia, and the atrocities of the civil war in Sierra Leone. It’s not a job most people would volunteer for, but Cruvellier, a gentle and philosophical soul, has something steely and relentless in him. If journalism were not so parochial, he’d be well known in this country.

Cruvellier sat through every day of Duch’s trial, and “The Master of Confessions” is a brilliant study in the mind of a zealous servant to a maniacal ideology. After his capture, Duch readily (perhaps too readily) acknowledged his guilt in sending thousands of Cambodians to their deaths. The voluminous prison archive survived to condemn him—this micromanager hadn’t thought to destroy it when he fled the Vietnamese Army. Now this once-obscure genocide has given the world perhaps the most complete testimony by a mass murderer in history. The trial—throughout which the defendant had the opportunity to speak at length—produced a detailed portrait of Cambodia under the Khmer Rouge: the blinding vision of a classless society, the bureaucratic machinery of torture and murder, the hunger, the paranoia, the endless denunciations as the regime began to eat itself. The country became a nightmare prison, and anyone unfortunate enough to end up inside—a Cambodian professor who returned from Europe to find out what had become of his family; a young adventurer from New Zealand whose boat strayed into Cambodian territory—had little chance of ever emerging.

Duch, in Cruvellier’s account, comes across as meticulous and intelligent, rather arrogant, indifferent to some witnesses and shattered by others, ready to confess, but, finally, incapable of fully coming to grips with the enormity of the crimes or his willing participation in them. When confronted with the killing of hundreds of children at his prison, Duch splits hairs: maybe some were smashed against trees, but none could have been thrown from the second floor. “The execution of children and babies was a part of his administration with which he wasn’t overly concerned,” Cruvellier writes. “It’s also one of the crimes with which his conscience struggles.”

Cruvellier turns the trial—at times, so monotonous that the unruly Cambodian crowds who came from around the country to watch were at risk of falling asleep—into a great drama. Duch’s “gaze is intense but strangely veiled, bright and glassy at the same time.” The lawyers reach rhetorical heights, then show themselves to be incompetent. The human-rights activists whose dedication helped make the trial possible “can be a hard-hearted bunch. Theirs is a world riven by bitter quarrels, scandal-mongering, jealous slander, and vindictive scheming.” In the end, after all the devastating words spoken by and about him, Duch remains impenetrable. “We cannot comfort ourselves by dismissing as deviants those men and women who perpetrated mass crimes in extreme political circumstances,” Cruvellier writes. “Duch is neither mentally ill nor a monster, and that’s the problem.”

We want an answer to the overwhelming question “Why?” The trial provides a narrative, a catharsis, and a ritual of justice. But nothing can satisfactorily explain what Duch has done, or tell us whether you and I would have done it in his place. Duch’s accomplice Mam Nai remains enragingly unapologetic. Thousands of perpetrators will never stand trial. Many of the top criminals, such as Pol Pot, have escaped through death. The families of the victims are haunted, destroyed. The gap between the two million dead and the elderly defendant with decayed teeth defeats any desire for retribution. Some wrongs are too great ever to be made right.

The sense of solidarity with victims in far-off, alien places is a fragile, flickering thing. After Cambodia, the years leading up to 2003 and Iraq were high points of consciousness—the years of Human Rights Watch and Amnesty International, the CNN effect, humanitarian intervention, the Responsibility to Protect. Today, after Iraq and so much else, we no longer know what to say or do when innocent people are being slaughtered somewhere. We no longer trust our own intentions and abilities, and rightly so. We have kept saying “never again,” but it keeps on happening, again and again—in Syria today, in the Central African Republic tomorrow. Without an idea of action, that sense of solidarity atrophies.

But this is the value of Cruvellier’s book, like the tribunal itself, like the commemorations this week of the twentieth anniversary of the Rwandan genocide, like the annual reading later this month of the names of the Jewish victims of the Holocaust. They make it a little easier to fight against what Cruvellier calls “the world’s propensity to forget, and against the impure pragmatism of the powerful.”

Photograph: Extraordinary Chambers in the Courts of Cambodia, Nhet Sok Heng/AP