One of the hard truths of human affairs is that diversity and democracy do not go easily together. In today's Middle East, as in Europe's not-so-distant past, the transition from authoritarianism to popular sovereignty seems to run through ethnic or religious purges. Worldwide, many of the models of successful democratic government are effectively ethno-states, built on past cleansings or partitions or splendid isolation. And in the West in recent years, both cultural fragmentation and mass immigration have revived authoritarian temptations.
This pattern runs deep in our species' history. A new paper from economists Oded Galor and Marc Klemp finds a strong correlation between diversity and autocracy in pre-colonial societies, with a legacy that also extends to today's institutions. The authors suggest authoritarianism emerges from both bottom-up and top-down pressures: A diverse society seeks strong central institutions for the sake of cohesion and productivity, and internal division, stratification and mistrust increase "the scope for domination" by powerful elites.
We in the United States like to think of ourselves as exceptions to this rule - and, notwithstanding the fate of the Native American tribes and the legacy of chattel slavery, we have been successful at combining republican self-government with racial and religious diversity.
But at the same time we aren't exactly governing ourselves via New England town meetings. As the US has become larger, more diverse and lately more fragmented, power has grown ever more centralised in Washington, and the face of that central government, the presidency, has accrued more and more authority. The caudillo-from-Queens style of Donald Trump is unique to the man himself, but it's also an outgrowth of trends that go back generations. We still have republican forms, but we also have a kind of elected emperor who presides over our enduring colour lines, our not-always-melted immigrants, our increasing mistrustful sects and tribes and classes.
The European Union doesn't have so singular a leader, but its ruling class is in a similar situation - they're the custodians of a diverse imperium, trying to preside over Greeks and Germans, Scandinavians and Sicilians, Christian natives and Muslim immigrants, while wielding powers that are at least one remove from democratic accountability.
This means that in understanding the challenge facing Western leadership, it's worth pondering the ways in which the world's authoritarian regimes interact with ethnic and religious diversity - exploiting it, managing it, or both.
In one common pattern, authoritarian rule evolves as a way for a majority or plurality group to hold power against the claims of diverse minorities, and to impose a kind of uniformity on weaker ethnic or religious groups. The Recep Tayyip Erdogan regime in Turkey and the Saudi monarchy's Sunni authoritarianism are obvious examples; as is the Han-Chinese chauvinism of China's Politburo, the Orthodox-Christian Russian nationalism of President Vladimir Putin, and many more.
In another pattern, an authoritarian leader - sometimes from a minority group himself - casts himself as a protector of diversity, promising to shield minorities who would be threatened should a majoritarian populism take power. This is the pattern of President Bashar al-Assad's rule in Syria, which has drawn support from his own Alawite sect as well as Syrian Christians and others fearful of what Sunni rule might mean for them. The Egyptian military regime, likewise, promises to protect urbanites and Coptic Christians from the Islamist order that democracy might usher in.
These patterns have echoes in our own imperial - er, presidential - politics. The coalition Mr Barack Obama built across two presidential elections united minority constituencies with the upper-class intelligentsia and pledged to champion their diverse interests against the remains of the country's white Christian heartland core. The Trump reaction was more Erdoganian or Putinesque, promising to protect a once-dominant majority, to restore its privileges and reverse its sense of cultural decline.
In Europe, meanwhile, the EU often seems to be run for the benefit of Germans at the centre and ethnic minorities at the periphery, favouring separatists and immigrants over old national majorities. The present populist surge is, in its turn, an attempt to establish a different dynamic between the continent's diverse factions, in which Germany has less power, more immigrants are turned away, and the old nations reassert themselves as centres of influence once more.
Neither continent is poised for a real slide into autocracy - I think! But on both, paradoxically, the cause of liberal order might be better served by leaders who took a slightly more imperial perspective - not in the sense of imposing policy at sword point, but in the sense of realising their societies are so diverse as to require a more disinterested kind of vision from their rulers.
Such a disinterested ruler - a good emperor, let's call him - would see a crucial part of his role as reassurance, recognising that in a diverse, fragmented and distrustful landscape, any governing coalition is going to look dangerous to those who aren't included in it. If he comes from a historically dominant group and speaks on their behalf, he needs to go out of his way to address the anxieties of minorities and newcomers.
If he's building a coalition of minority groups, he needs to reassure the former majority that the country of the future still has a place for them. Whatever the basis of his power, he needs to be constantly attuned to the ways that diversity, difference and distrust can make political conflict seem far more existential than it should.
The last two US chief executives recognised that they needed to make efforts along these lines, but with exceptions - Mr George W. Bush after Sept 11, Mr Obama in his 2008 campaign - they were not particularly successful. In Mr Obama's case, his White House failed to grasp the feeling of abandonment and crisis in the white heartland, and the extent to which that feeling was creating a new identity-based voting bloc. He also failed to grasp how threatening the regulatory state's enforcement of liberal sexual norms was to religious conservatives, how much it made them feel like strangers in their own country.
From that alienation and fear came Mr Trump, who is barely even trying to reach out and reassure, to make his nationalism seem larger than just white identity politics, to make the groups who feel afraid of his administration sense that he has their anxieties in mind. There might be a form of nationalism that helps bind a diverse society, but Mr Trump's seems more likely to bind a "real American" ex-majority in opposition to every other race and faith and group.
His eventual successor, liberal or conservative, should not seek to learn from Mr Assad or Mr Erdogan or Mr Putin. But he (or she) might learn something from an earlier age's custodians of diverse, fragmented societies - from monarchies like that of the Austrian Hapsburgs, in particular, that worked to contain and balance religious and ethnic divisions, to prevent disintegration and forestall totalitarianism, and might have succeeded longer absent the folly of 1914.
If we're going to have an imperial presidency, we should want a president who thinks less like a party leader and more like a good emperor - who doesn't just divide and conquer, but who tries to make all his empire's many peoples feel like they're safe and recognised and home.