Racism, one of the ugliest features of human society, is also among its most resilient.

When the African American scholar W. E. B. Du Bois wrote, at the very beginning of that century, that the ‘problem of the twentieth century is the problem of the color-line’, he summed up a core reality of the next hundred years. But perhaps not even he could have imagined that, two decades into the twenty-first century, it too was on its way to becoming a century whose problem would be ‘the color-line’. Focused on the ‘relation of the darker to the lighter races’, Du Bois could not have expected that the worst racially motivated slaughter during the century would be visited by whites on other white people, not on people of a darker complexion. The Nazi genocide slaughtered six million Jews and between eight and ten million Slavs simply because they had been born into the ‘wrong’ ethnic group. (If we calculate deaths as a proportion of population size, the most brutal mass murder of the modern era was that of ten million Congolese people at around the time that Du Bois’ observation appeared in print; of course, the sheer number of Nazi victims was greater.)

While the Nazis slaughtered Slavs and Roma people too, racism directed at Jews was a core element in their ideology of the Aryan ‘master race’. The prejudice was not new; active hostility to Jews stretches back to the founding of Christianity and was hardened by the emergence of absolutist states in the Late Middle Ages. But Jewish identity is complicated by the fact that Jews are both adherents of a religion and members of an ethnic group. This is not unique; Sikhs are another well-known example of a religious group which is also an ethnic group. But it is unusual.

Until the French Revolution and the Enlightenment, these two identities were fused (for reasons that will be discussed in chapter 4), and all Jews were effectively forced to adhere to their religion by the reigning authorities. When Jews were allowed to choose whether to practise their religion, those who chose not to were still regarded as ethnically Jewish. This made Jewish identity more complicated than that of most other religious or ethnic groups.