ETHNIC NATIONALISM, the desire for one’s race to be pure and to exclude others from one’s claimed territory, can in extreme cases cause ethnic cleansing, as we saw in the Yugoslav civil wars of the 1990s. To think of ourselves as having a monochrome nationality, a primary marker to distinguish ourselves from others, can result in an antagonistic view of identity—British versus Irish, Croat versus Serb, and so on. This can lead to nations, in the name of a false sense of historical coherence, behaving badly. Engels referred to the “unhistoric races” of Europe, peoples who had hardly ever ruled…