But as McNeill suggests, nationalism can be understood as a unifying alternative to a society built on polyethnic hierarchy, in which a series of hereditary ethnic castes live together in uneasy peace, usually with some dominating the others. It is polyethnic hierarchy that has been the norm throughout modern history, not national unity or egalitarian pluralism. One could argue that the dream of pluralism without hierarchy is at least as chimerical as that of an egalitarian nationalism built on the melting and fusing together of once-distinct groups, if not far more so. McNeill’s stylized history gives us a sense of what we’re up against as we try to build decent and humane societies amidst entrenched ethnic divisions, and why so many modern thinkers have embraced the politics of national unity.

Until around the mid-18th century, McNeill argued, ethnic-political unity in major civilizations from Europe to the Mediterranean to the Asian steppe “was often illusory and always fragile.” The 19th-century ideal of the homogenous nation-state, which took off with particular intensity in revolutionary France, was something of a historical anomaly, one that the epic demographic and ideological shifts of the bloody 20th century would begin to erase. Today we may find ourselves at another major turning point in the relationship between state and ethnos.

Read: The everyday psychology of nationalism

McNeill cites three factors that obstructed the formation of stable and cohesive national identities for much of recorded history. First, in the premodern period, the continuous conquest and reconquest of vast territories by rival bands of military nomads competing for resources ensured continuous ethnic mixing and upheaval, especially in major population centers. “The rise of nomadry as a way of life,” McNeill writes, “acted on the peoples of Eurasia like an enormous gristmill, grinding the peasant majority exceedingly hard since it was they who suffered plunder and paid taxes, sustaining their military masters and all the other occupational specialists who congregated in cities and maintained the arts and skills of civilization.”

The second fact of premodern and early-modern life that forced urban ethnic mixing was the prevalence of fatal infectious diseases in populous areas. Mortality rates were so high that cities were not able to sustain their population through reproduction alone, and while some of the labor shortage could be made up by organic migration from nearby rural areas, political elites more often than not resorted to the importation of ethnic “others” as slaves, helots, or serfs from their imperial peripheries. Even the classical city-states of Athens and Rome, sometimes regarded as model egalitarian liberal republics, would soon transform into mighty empires whose capitals were flooded with foreign-born ethnic outsiders who were politically and economically subordinate to the dominant class of Greek or Roman citizens.