“A man’s home is his castle,” Joseph ­Goebbels told the League of Nations in 1933, so “we will deal as we see fit” with our various “opponents” and, in particular, “our Jews.”

At the time, Goebbels’s view was almost universally shared. However shocking, detestable and morally indefensible it might seem today, no one dreamed of contesting it then. ­Sovereignty — a term that muddled people’s right to decide for themselves and the right of despots to decide for their people — was the first and last word in international relations.

If the same cannot be said today, if dictators are no longer seen to hold the power of life or death over their subjects, if the archcriminals of Cambodia, Sudan and Rwanda are ­indicted and sometimes even punished, in short, if the idea of international justice has gradually gained a semblance of meaning, we owe it to two ideas, or more precisely two concepts — as well as to the two men who brought them to life: Hersch Lauterpacht for the concept of the crime against humanity and Raphael Lemkin for that of genocide. Philippe Sands, a professor of law at University College London, recounts the life and work of both men in “East West Street: On the Origins of ‘Genocide’ and ‘Crimes Against Humanity.’ ”

Sands begins by drawing distinctions between the two concepts. He is careful to show how, despite their complementarities, the two rest on different, even opposing, notions of rights. One is ­rooted in individual rights, the other in the rights of groups. One places at the top of the scale of offenses those perpetrated on individual men and women, the other the intention to annihilate the population or community from which those individuals spring.

The bulk of Sands’s book deals with the parallel histories of two Jewish thinkers born at about the same time (1897 and 1900) in the same disputed patch of Eastern Europe (now parts of Ukraine and Belarus). His protagonists are indefatigable champions of rights who fought for their entire lives, indeed to their last breath, to produce a philosophy that led to the Nuremberg trials and then, decades later, to the possibility that not all of the mass murderers of our time will pass away peacefully in their sleep.

Sands proceeds in the manner of certain historians of science who, in recounting a discovery, begin by inviting the reader to grasp the weight of the obstacles that had to be overcome to arrive at the advance. In this case, those obstacles were the pusillanimity of Western diplomats; the presentiment of the Soviets that the nascent international laws might one day be turned against them; and the inconceivability in every legal tradition of the very idea that an individual, a subject, might have rights capable of being ­asserted against a state.

But he also works in the manner of the authors of thrillers, who, to convey the motives of their characters, present clues that are sometimes seemingly insignificant along with others that are immediately enlightening: a name in an old directory; a faded photo; an illegible address on a piece of cardboard; an incomprehensible but unyielding silence; a classroom in a Polish city, the walls of which echo with the anger of a law student (Lemkin) challenging his professor about the case of a young Armenian charged with the murder of one of the perpetrators of the first modern genocide; or the novelistic situation of a law professor (Lauterpacht) arriving in Nuremberg with the certitude that his own family was exterminated by the very man he has come to charge and ­­judge.

Discovering that his heroes, Lemkin and Lauterpacht, lived and studied in the city of Lviv and attended lectures by the same law professor, Juliusz Makarewicz, realizing, in other words, that all of the tracks of his history lead back to the same Galicia where for 70 years the living have been surrounded by ghosts, by the memory of hundreds of thousands of exterminated but tombless Jews, Sands becomes an archaeologist digging into bloody ground. He excavates an Atlantis without marker or memorial: the remains of a train station; the empty lot where a synagogue stood; an abandoned storefront; a former site of the university where the arguments were honed that would one day prevent Nazi war criminals from falling back on the precedent of British colonialism or of the murder of African-Americans by the Ku Klux Klan; the site of the Janowska camp, where Jews who had been deemed unsuitable for forced labor were sorted before being sent on to the Belzec extermination camp; another photo; a diploma. Again and again, Sands interlaces the history of the crime with the relentless effort of Lemkin and Lauterpacht to grasp its scope and to name it — an effort almost superhuman because they were working as the crime itself was unfolding.

In Sands’s history, as in all great novels, we encounter characters who, though seemingly secondary, are essential to the plot. One such is Hans Frank — patron of the arts, governor-general of occupied Poland, author of crimes against humanity and perpetrator of genocide — whose case will eventually be heard at Nuremberg, where he will be sentenced under an idea that is simultaneously receiving its ultimate form. (The effect was of a circle that traditional justice might call vicious but that probably constituted the virtue of the new justice then taking shape.)

Another of Sands’s people is the war correspondent and novelist Curzio Mala­parte, who watches Frank raise his Bohemian crystal wineglass and say, without blinking, “You may drink without fear, my dear Malaparte; this is not Jewish blood.” A third is Elsie Tilney, righteous among the righteous, a sublime and modest heroine, who, to save a Jewish child, undertakes a long, perilous and most enigmatic voyage.

And all the while Sands works in the way of artists like Filippo Lippi, who painted himself into the corner of his “Coronation of the Virgin” and “The Funeral of Saint Stephen.” Much of the strange beauty of this book turns on the fact that the author finds his own genealogy to have its roots in the city of Lviv. Leon Buchholz, Sands’s grandfather, might as well have been related to his two main characters. And what emotion we feel when it dawns on Sands, a specialist in international and human rights law, that he too is linked to some of the great trials at the end of the 20th century that saw the posthumous victory of Lemkin and Lauterpacht.

The result is a narrative, to my knowledge unprecedented, in which the reader observes the life and work of two ordinary men drawn by unwavering passion and driven very nearly insane by the griefs and the hopes bequeathed to each of them by the tumult of an unreasonable time. But that very madness and their thirst for truth and justice gives them access to the universal, enabling them to lay the cornerstones of the cosmopolitan justice that the world had been waiting for since Kant.

And finally, taking into account the old principle (Spinoza’s this time) that one never makes such good use of a tool as when one knows the secrets of its manufacture and thus its history, we have in Sands’s “East West Street” a machine of power and beauty that should not be ignored by anyone in the United States or elsewhere who would believe that there are irreparable crimes whose adjudication should not stop at the border.

“I’ve read your book,” the 44th president of the United States is reported to have said to Samantha Power, one of his key advisers on genocide and crimes against humanity. Barack Obama and his successors would be well advised to move to the top of their reading lists this account of the birth, amid the darkest conceivable shadows, of an unprecedented body of rights-based law, whose application has scarcely begun.