Three things seem to me necessary to explain the extreme violence of the twentieth century, and in particular why so much of it happened at certain times, notably the early 1940s, and in certain places, specifically Central and Eastern Europe, Manchuria and Korea. These may be summarized as ethnic conflict, economic volatility and empires in decline. By ethnic conflict, I mean major discontinuities in the social relations between certain ethnic groups, specifically the breakdown of sometimes quite far-advanced processes of assimilation. This process was greatly stimulated in the twentieth century by the dissemination of the hereditary principle in theories of racial difference (even as that principle was waning in the realm of politics) and by the political fragmentation of ‘borderland’ regions of ethnically mixed settlement. By economic volatility I mean the frequency and amplitude of changes in the rate of economic growth, prices, interest rates and employment, with all the associated social stresses and strains. And by empires in decline I mean the decomposition of the multinational European empires that had dominated the world at the beginning of the century and the challenge posed to them by the emergence of new ‘empire-states’ in Turkey, Russia, Japan and Germany. This is also what I have in mind when I identify ‘the descent of the West’ as the most important development of the twentieth century. Powerful though the United States was at the end of the Second World War — the apogee of its unspoken empire — it was still much less powerful than the European empires had been forty-five years before.