half-century were either ideological, exemplified by the Cambodian case, or retributive, as in Iraq [Saddam Hussein’s 1988–91 campaign against Iraqi Kurds]. The scenario that leads to ideological genocide begins when a new elite comes to power, usually through civil war or revolution, with a transforming vision of a new society purified of unwanted or threatening elements. Retributive geno-politicides occur during a protracted internal war . . . when one party, usually the government, seeks to destroy its opponent’s support base [or] after a rebel challenge has been militarily defeated.169
The decline of genocide over the last third of a century, then, may be traced to the upswing of some of the same factors that drove down interstate and civil wars: stable government, democracy, openness to trade, and humanistic ruling philosophies that elevate the interests of individuals over struggles among groups.
For all the rigor that a logistic regression offers, it is essentially a meat grinder that takes a set of variables as input and extrudes a probability as output. What it hides is the vastly skewed distribution of the human costs of different genocides—the way that a small number of men, under the sway of a smaller number of ideologies, took actions at particular moments in history that caused outsize numbers of deaths. Shifts in the levels of the risk factors certainly pushed around the likelihood of the genocides that racked up thousands, tens of thousands, and even hundreds of thousands of deaths. But the truly monstrous genocides, the ones with tens of millions of victims, depended not so much on gradually shifting political forces as on a few contingent ideas and events.
The appearance of Marxist ideology in particular was a historical tsunami that is breathtaking in its total human impact. It led to the dekamegamurders by Marxist regimes in the Soviet Union and China, and more circuitously, it contributed to the one committed by the Nazi regime in Germany. Hitler read Marx in 1913, and although he detested Marxist socialism, his National Socialism substituted races for classes in its ideology of a dialectical struggle toward utopia, which is why some historians consider the two ideologies “fraternal twins.”170 Marxism also set off reactions that led to politicides by militantly anticommunist regimes in Indonesia and Latin America, and to the destructive civil wars of the 1960s, 1970s, and 1980s stoked by the Cold War superpowers. The point is not that Marxism should be morally blamed for these unintended consequences, just that any historical narrative must acknowledge the sweeping repercussions of this single idea. Valentino notes that no small part of the decline of genocide is the decline of communism, and thus “the single most important cause of mass killing in the twentieth century appears to be fading into history.”171 Nor is it likely that it will come back into fashion. During its heyday, violence by Marxist regimes was justified with the saying “You can’t make an omelet without breaking eggs.”172 The historian Richard Pipes summarized history’s verdict: “Aside from the fact that human beings are not eggs, the trouble is that no omelet has emerged from the slaughter.”173 Valentino concludes that “it may be premature to celebrate ‘the end of history,’ but if no similarly radical ideas gain the widespread applicability and acceptance of communism, humanity may be able to look forward to considerably less mass killing in the coming century than it experienced in the last.”174
On top of that singularly destructive ideology were the catastrophic decisions of a few men who took the stage at particular moments in the 20th century. I have already mentioned that many historians have joined the chorus “No Hitler, no Holocaust.”175 But Hitler was not the only tyrant whose obsessions killed tens of millions. The historian Robert Conquest, an authority on Stalin’s politicides, concluded that “the nature of the whole Purge depends in the last analysis on the personal and political drives of Stalin.”176 As for China, it is inconceivable that the record-setting famine of the Great Leap Forward would have occurred but for Mao’s harebrained schemes, and the historian Harry Harding noted of the country’s subsequent politicide that “the principal responsibility for the Cultural Revolution—a movement that affected tens of millions of Chinese—rests with one man. Without a Mao, there could not have been a Cultural Revolution.”177 With such a small number of data points causing such a large share of the devastation, we will never really know how to explain the most calamitous events of the 20th century. The ideologies prepared the ground and attracted