In Phnom Penh, between 1975 and 1979—the years of Khmer Rouge terror that Cambodians often refer to simply as Pol Pot Time—a former schoolteacher named Kaing Guek Eav, better known as Duch, orchestrated the torture and execution of at least twelve thousand men, women, and children. The Khmer Rouge murdered close to two million Cambodians in that period. That’s what their particular brand of Communist revolution amounted to: killing. Duch wasn’t one of the masterminds, but he was their zealous servant, and he was entrusted with the command of S-21, the prison where Khmer Rouge cadres were sent to be purged. The purges were constant. While ordinary Cambodian civilians were killed on an industrial scale and without ceremony, it was Duch’s mission to insure that everyone held at S-21 was broken down until they confessed to counter-revolutionary crimes—working for the C.I.A., say, or for the K.G.B., or for both, even though most prisoners had never heard of either agency before Duch’s torturers went to work—and then he had them slaughtered.

Duch didn’t expect to survive the revolution: he had sent most of his own mentors to their deaths, and, by the logic of S-21, his time, too, would come to confess and be condemned. But, before that happened, a renegade group of Khmer Rouge officers backed by Vietnam drove the Pol Potists from power. Duch had kept excellent records of his work; unlike nearly everyone else in the revolutionary hierarchy, he had failed to destroy them in the end. So, in the nineteen-nineties, when it was discovered that he had survived, there was no denying his crimes. By then, he had converted to Christianity, and he said he repented of his career as a Khmer Rouge murderer. That made him singular. In 2009, thirty years after he fled S-21, he was the first Khmer Rouge figure to be brought to trial before a newly established, U.N.-run tribunal in Phnom Penh. And, luckily, Thierry Cruvellier, who has spent the past fifteen years observing international war-crimes prosecutions more closely than any other reporter and writer, was there for every day of Duch’s trial.

Duch was an intensely engaged participant in his own trial, and Cruvellier gave his exceptionally fine portrait of the man and his judgment the double-edged title “The Master of Confessions.” When the book originally appeared, in France, Cruvellier was immediately acclaimed as a master in his own right—a deeply informed and deeply thoughtful observer of the legal, political, moral, and psychological complexity of his subject. He is an elegant, understated writer, with a keen and rigorous intellect, and a wry, quiet wit. We first met and became friends in May of 1995, in Kigali—his first book, “Court of Remorse,” drew on the five years he spent at the U.N.’s Rwanda tribunal—and, shortly after the American publication of “The Master of Confessions,” last month, we began this e-mail interview.

You’re the only journalist who has attended all the post-Cold War international tribunals. You’ve spent years watching these trials. What drew you to them? What kept you going back? How has your view of them evolved?

I was drawn to war-crimes justice because of Rwanda. The 1994 genocide was a defining event for our generation. I began working in Rwanda in the immediate aftermath, so covering the trials seemed like a logical way to keep working on this event. And I quickly realized how fascinating these trials could be, at so many different levels: historical, political, diplomatic, legal, psychological, philosophical. My great interest in the trials was as a window, on the one hand, into our human condition in extreme circumstances, and the choices individuals make (or lack) in such situations; and, on the other hand, into the historical complexity of the dynamic of genocide at the central level. A courtroom presents an extraordinary opportunity to observe such out-of-scale human tragedy and its political background, through the reasonably accessible life and actions of an individual and a wonderfully varied cast of supporting characters. For a writer, this is incredibly rich territory—just as long as you are ready to bear the boredom, the mediocrity, and the disillusionment.

The good thing about working on the same story over a long period of time is that you tend to get better at what you’re doing. The bad thing about working too long on mass violence is that it doesn’t make you a happier man.

You are often sharply critical, even damning, in your account of these courts. Are we better off with them than we were without them?

It’s true that I can be very harsh on these institutions and on those who run them. Historically, it is understandable that these judicial experiments had to be tested and developed, and some have been more convincing than others, at different stages. But, in the beginning, when the Rwanda tribunal got under way, most of us were quite naïve; we thought that this was our Nuremberg. In fact, the quality, efficiency, and sense of purpose of international tribunals—and of the International Criminal Court, which is their heir—have deteriorated steadily over the past decade.

The Duch trial had its shortcomings, of course. The prosecutor’s office, for instance, was very weak. Yet it remains by far one of the most complete, fair, and valuable trials I have covered. The “mixed” nature of the court, with equal participation of Cambodians and internationals; the fact that it took place in the country, in Phnom Penh; and the fact that victims were accepted as full parties to the proceedings made this trial much more relevant and “real” than those held in The Hague.

Journalists and historians lay a lot of blame for the destruction of Cambodia and Rwanda and the former Yugoslavia on the actions and inactions of major Western powers. But tribunals never consider such outside responsibility. Do you think that they could or should?

Genocide is primarily a national affair. The way Rwandan Hutus and Khmer Rouge cadres decided to instigate or join the killings, or the way top leaders orchestrated, instigated, or gradually got into the mechanics of mass murder are not decisively linked to the support they may have got from outside. The responsibilities of foreign governments—and of France in Rwanda, in particular—seem to work at a different level than what is defined by international criminal law. Technically, criminal courts can only deal with individual responsibility, which makes it highly difficult to link a foreign state to the crime. Ultimately, the genocide is conceived and committed by nationals.

Then, of course, there’s a more embarrassing reason these courts don’t go after foreign responsibility: judges and prosecutors don’t want to get into trouble with permanent members of the U.N. Security Council or Secretariat, which pays most of their salaries. It’s an obvious weakness of these tribunals, but perhaps it’s just not their function. Their credibility problem may lie much more in the poor quality of the investigations, and in the fact that only the weak are prosecuted.

You make the key point that the Duch trial was the first international tribunal case to address the crimes of Communism. The Rwanda and Yugoslavia courts, like the prosecutions at Nuremberg and Tokyo, dealt with crimes of ultra-nationalist regimes, which you identify as ideologies of the right. Only the Cambodia tribunal has addressed the crimes of the left, and you say that made human-rights lawyers notably uneasy. You say they had great difficulty addressing the connection between Communist ideology and systematic mass murder. You say that much of the tribunal crowd preferred to imagine the Khmer Rouge as noble until it went awry and became vile—and that some were outright fellow-travellers. For instance, the woman hired by the U.N. to handle Khmer Rouge victims at the Duch trial was an unrepentant Maoist. Why was that? And how did this sympathy for the left affect the general atmosphere of the trial?