How much murder is too much? Ethnic cleansing is a crime, but what qualifies? Does slaughtering a village count, or do you have to lay waste to a larger polity, perhaps with some torture thrown in? How many people do you have to kill before graduating from mere mass murder to full-on genocide?

The legal answer, strangely enough, is zero. In Ben Kiernan’s “Blood and Soil” (Yale University Press, 606 pages, $40), a meticulous new study of this most slippery of criminal categories, he points out that the standard definitions of genocide — those offered by the U.N.’s Genocide Convention and International Criminal Court Statute — require not even a single death, or indeed any physical harm at all. In fact, the génocidaire need not even target a whole ethnic group. To win a place in the defendant’s chair — or a mention in Mr. Kiernan’s book — requires only the attempt to cause that group “serious mental damage.” The extermination of European Jewry counts, but so does a single British colonial officer’s efforts to take away an Australian aboriginal child from her parents, involving, as it did, the intent to “breed out the color.”

These legal standards are in a way too narrow, since surely the cold-blooded murder of a whole village is vile enough to merit opprobrium, regardless of whether it fits a strict definition of “genocide.” In “Blood and Soil,” Mr. Kiernan highlights the contrast between our conventional and our legalistic definition of genocide by choosing case studies that, with few exceptions, attain the highest standard of vileness. And yet, the definitions offered by the UN and the ICC do say something meaningful about the nature of genocide: Even when physical facts (such as the number dead, and at whose hands) are freely acknowledged, the intent of the perpetrators is actually the key issue.

This 600-page volume plumbs the mens rea of the ethnic cleanser, from the Punic Wars to Darfur. The exhibits range from the well-known (Tutsis in Rwanda, Jews in World War II) to the more obscure (the brutality against the Herero by colonists in 1904 German Southwest Africa) to the forgotten (the Chams whose 15th-century empire was annihilated by the Vietnamese). The chapters on the least known of the genocides offer particular value as introductions to overlooked regional histories, and the material on the Nazis and Ottoman Turks nicely situate both those groups within larger contexts of ethnic violence. Each case is written sharply enough to escape the aroma of potted history that sometimes afflicts comparative studies of this type or political accounts, such as Samantha Power’s “A Problem from Hell: America and the Age of Genocide.”

But the most unsettling aspect of “Blood and Soil” is its repeated emphasis on agriculture as a historical driver for genocidal impulses. In nearly every genocide Mr. Kiernan cites, the aggressors prided themselves on their prowess as cultivators and denounced their victims as inferiors because they lived in cities or were traditionally pastoral. Europeans all came from civilizations built on farming. The groups they targeted never farmed with the same enthusiasm: The Aztecs had small communal plots, and the Herero lived as nomads. The word “savage,” Mr. Kiernan points out, shares its roots with “sylvan,” meaning “of the forest,” and therefore not part of a settled society that survives through farming.

Mr. Kiernan argues that the rate of genocidal violence of agrarian societies against non-agrarian ones is high enough that we should consider cultivation — and especially the romanticization of farming, coupled with utopian or religious zeal — a leading indicator of genocide. As a purely practical matter, the development of agriculture is a prerequisite to having enough idle and stationary time to craft the tools and strategy for genocide on large scales. And historically, the pattern simply seems to fit. During the Punic Wars, Cato the Censor – who ended his every Senate speech with a plea for the obliteration of Carthage — whipped up his hatred by idealizing the Roman gentleman farmer, in contrast to the moneylenders and merchants of Carthage. Echoes of the same complaint provided inspiration to the grisly conquista by Spanish settlers who thought native Mesoamericans lacked nobility in their inability to subdue the land through cultivation. Hitler, too, idealized the German farmer, and Mr. Kiernan quotes a line of Himmler’s to the effect that the flower of German military valor grew in the fields of the Fatherland, and that cowards all hailed from the cities.

It’s possible to lapse into determinism of one kind or another when examining genocide, and invoking agricultural lebensraum as motivation for mass killing, as Mr. Kiernan does, comes close. Explaining mass murder through protein scarcity or overpopulation seems inadequate at best, and at times the explanations themselves smack of racism, due to their implication that among certain races, but not among others, a natural reaction to overpopulation is to hack off a neighbor’s limbs. But Mr. Kiernan is more disciplined than the determinists. In his model, genocides do not grow out of resource scarcity. Rather, they happen as a matter of farmer-on-nomad violence, and agrarian idealism — instead of any actual pressure on the land — is merely one frequent factor.

Moreover, unlike others, such as Jared Diamond, he treats the perpetrators as agents and allows them to name their own reasons for genocide. Those perpetrators are often startlingly honest, and unexpectedly articulate in their fetish for land cultivation and for national soil. Gen. James Wilkinson, commander of the American Army under Jefferson, wrote candidly that if American Indians could not be induced to start using the land properly by farming like Europeans, “the seeds of their extermination, already sown, must be matured.” When the aging leaders of the Khmer Rouge — a movement that murdered millions in service of a nationalist agrarian ideal — finally came in from the jungles in the late 1990s, they apologized for excess loss of life during their rule, but they also took care to regret the damage done to animals and land.

This is grim stuff, made grimmer still by the book’s implication that genocide is a normal feature of history, and that it seems to feed on itself. Mr. Kiernan points out that perpetrators of genocide often look to previous genocides as models. In the language of his battlefield orders, Hernán Cortés showed evidence of familiarity with Julius Caesar’s ruthless and genocidal campaigns against northern European tribes. Hitler famously asked whether anyone remembered the Armenian genocide. Somewhat less famously, the Third Reich profited from Germany’s experience in Southwest Africa years before. In the concentration and extermination of the Herero and Nama, one sees not only models for the spurious racialism of Nazi Germany but for the death camps themselves. Indeed, one of the most disturbing aspects of genocide is that its successful and unpunished commission bequeaths to future generations — even generations centuries hence — the belief that mass killing is a legitimate option, and that here are indeed a few ways in which it can be done.

Mr. Kiernan hopes that his book will help identify and prevent future genocides, and he briefly addresses modern instances of genocide, such as Darfur, as well as the question of whether violent strains of Salafi Islam contain the seeds of a genocidal movement. He thinks they do. (Curiously, neither of these instances fit his model of agricultural chauvinism of genocides past. The losing side in Darfur consisted of cultivators, and the Janjaweed are mostly pastoralists.)

These modern cases demonstrate some of the limitations of the analysis of “Blood and Soil”: although both the Janjaweed and Al Qaeda show signs of genocidal intent — and indeed actual genocide — labeling them does little to help us deal with them. Whether something is “genocide,” and whether it follows the intriguing historical patterns that Mr. Kiernan identifies, is not really the point when we consider taking immediate action. The slogans of those who want to “stop the genocide” invariably conceal grand complexities, such as the long civil war in Darfur, as well as grand simplicities, such as the wickedness of radical Islam. The relevant facts to acknowledge are the complexity and the wickedness: Whether there is also genocide, agriculturally-induced or otherwise, is, for all but the historian and the lawyer, a moot point.

Mr. Wood is a staff editor at the Atlantic Monthly.