The press hailed Tehlirian as a hero. But legally his situation was a disaster. Whereas Pasha never had to face a court, Tehlirian was put on trial as a common murderer. In the event, he was set free, but only because a Berlin court was willing to pretend that he had acted under “psychological compulsion.”

Raphael Lemkin, then a young law student at the University of Lwow, wasn’t satisfied with that subterfuge. He was revolted that somebody who had “upheld the moral order of mankind” should be “classified as insane.” And so Lemkin set out to persuade the world to adopt a law against the kind of “racial or religious murder” that had claimed the lives of Tehlirian’s relatives.

Against the odds, he succeeded.

“Totally Unofficial,” Lemkin’s posthumous autobiography, tells the story of his remarkable achievements. Born in 1900 to Polish-Jewish parents of modest means in a remote corner of Western Ukraine, his rise was meteoric. In short succession, he established himself as a prominent lawyer in Warsaw, escaped the Nazi invasion of Poland, coined the term “genocide,” served as an adviser to the U.S. War Department and became a law professor at Yale.

 

Thanks to Lemkin’s efforts, on Nov. 9, 1948, the 10th anniversary ofKristallnacht , the United Nations General Assembly unanimously adopted a “Convention on the Prevention and Punishment of the Crime of Genocide.” Cleaving closely to his proposal, it described genocide as “acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such.”

The horrors of the Holocaust had helped sway world opinion in favor of Lemkin’s cause. And yet he emphasized that—from Nero’s persecution of the Christians to the Mongols’ massacres of Eastern Europeans in the 13th century—genocide had occurred many times throughout history. It was at his insistence that the U.N.’s definition covered all cases—past, present or future—in which an ethnic or religious group was marked out for destruction.

Thanks to this adaptability, the term has gained lasting political as well as legal relevance. In the decades since Lemkin died of a heart attack in 1959, the term he invented has become the focus of a strange tug of war: Activists hope that the powerful label of genocide might move reluctant publics to stop atrocities; politicians fear that it could force them into costly foreign adventures or preclude negotiated settlements. In cases like Darfur, the question of whether given atrocities amount to “genocide” now plays a key role in determining how the international community will act.

Unfinished at his death, and published now for the first time, Lemkin’s autobiography gives a detailed account of his tireless advocacy. It will prove useful to generations of historians. But, like most autobiographies by historical figures, it also aims to cast its protagonist in a flattering light. By that metric, it is at best a mixed success.

“Totally Unofficial” suffers from big chronological jumps and uneven prose. While Lemkin is candid in parts, he just as frequently veers into the smug or self-righteous. Most of his contemporaries at the U.N. respected him; few found him winning. His autobiography makes it easy to see why.

The truth is more complicated. Lemkin was clearly a man of rare talents and single-minded devotion. To further the “lifesaving idea” for which, he believed, providence had chosen him as a “messenger boy,” he remained single, gave up a lucrative legal career and literally worked himself to death. Down to the details—like his poverty and his lifelong impatience with small talk—he makes for an excellent secular saint.

And yet his influence may not have been as transformative as he thought. The genocide convention would never have passed if it hadn’t been conformable to the interests of contemporary superpowers. Locked in a battle for ideological supremacy, the U.S. and the Soviet Union had strong reasons of their own to play to world opinion by condemning genocide. That also explains why Lemkin’s star quickly faded when he began to advocate for an international court to prosecute state officials for war crimes. While the great powers were happy to pay lip service to his lofty ideals, they were unwilling to compromise their sovereignty.

In the end, then, Lemkin doesn’t quite fit the role of the extraordinary individual bending history to his will. His life is interesting in an altogether different way: It is emblematic of both the ample promise and the real disappointment of international law.

In Lemkin’s own words, the point of the genocide convention had been nothing less than to be “a starting point for a new conscience.” Over time, he hoped, “a combination of punishment and prevention” would help to avert atrocities. Today, well-funded NGOs raise the alarm as soon as genocide looms in any part of the globe. Under Mr. Obama, the White House has even instituted an Atrocities Prevention Board. (Its first head: Samantha Power.)

But atrocities persist. Plenty of mass murderers remain at large. In recent years, a number of countries have agreed for the International Criminal Court to prosecute their citizens for war crimes, including genocide. But in reality only the genocidal leaders of small powers need to fear justice. Were Tehlirian alive today, he would have as much reason to become a murderer as he did in 1921.

Mr. Mounk’s “Stranger in My Own Country: A Jewish Family in Modern Germany” will be published in January.

A version of this article appeared July 24, 2013, on page A13 in the U.S. edition of The Wall Street Journal, with the headline: The Father Of Genocide.