Aside from the lesson contained in its title, Samuel Huntington makes the worrisome (and probably true) prediction that religion will re-emerge as an important force in international politics in the near future.*
The book is very much a product of its decade - written by an older American scholar in the 1990s,** immediately after the end of the Cold War - and contains frankly superficial compilations of historical and cultural detail. But he does give several supporting arguments for his thesis, all of them relating to two observations: modern states engage in wars for the people, not wars for kings; and that religion is one of the central defining characteristics of civilizations and therefore of individuals' identities. As history has progressed, civilizations are in contact with their neighbors more and more - and the human tendency to define oneself in opposition to the Other comes forward. We've gone from occasional trade caravans received only in royal palaces and seen by few, to universal social media - before, the Other was a rumor. Now, the Other is constantly in your face (even if it's a domestic Other - more on this later) and social media has produced a status monoculture. This gives rise to the idea captured in the book's title and in particular bloody border of Islam that is the best-known takeaway from this book. It's worth stressing that he wrote this book five years before September 11th.
But Huntington's argument would seem to apply to domestic politics just as well. Many Americans seem not to know what their political values are, other than if it pisses off the opposition, it must be good. And when the opposition has built a system that seems rigged for them to succeed, exalts their status, and denigrates yours, your identity (and your ability to define it against the Others) is in crisis. At civilizational borders you can just do your best to avoid the Others, but if you're in the same country there is no such strategy. Brexit and the election of Trump may both have been driven in part by basic threats to identity, exacerbated by class boundaries that are causing a realignment in the liberal and conservative parties on both sides of the Atlantic.
How so? Both of these stories are about a revolt by socially conservative, poor, ethnic-majority people living outside the successful metropolitan centers, against the professionals in those metropolitan centers who have run the show. (This is what exposes the cracks in both the liberal and conservative coalitions; see #4 here.) Professionals define themselves by their profession and the abstract principles that enable their profession (globalism prominent among them). In the 21st century, that's a winning strategy. But the concrete thinking authority-fearing folks in the interior who just want to raise a family and enjoy country life? They're no longer safe from the reach of international competition. They used to be comfortable, not constantly aware that a world existed beyond their communities, and they identified strongly with those communities. Maybe they weren't rich, but they knew who they were, and if anyone looked down on them, at least they weren't constantly reminded of it.
But now after we've seemingly finished cognitively sorting ourselves geographically, the people in San Francisco and London are unambiguously more materially successful, all the while demonstrating what looks to the folks back home like flagrant disloyalty to the homeland and (even worse!) looking down on (insert rural province here) because of their loyalty and respect for authority. From the Trump/Brexit crowd's perspective, the status monoculture is inescapable, and it's upside-down, with them at the bottom. Whether or not the provincial folk were starting the next Google, what was important was raising a good family in their town and being considered good folk by the people they knew. Now that's being taken away, with all the psychological impact of loss of meaning you might expect (e.g. chronic unemployment and the opioid epidemic). Another telltale of how the modern classes define themselves is the effect one's class has on ethnicity. The salience of ethnicity decreases among professionals, and increases among the loyalist left-behinds. (In the U.S., witness the high rate of interracial marriage among physicians and engineers; and in fact, white-Asian marriages produce wealthier households than either Asian-Asian or white-white.)
Huntington notes that the last four centuries of Western history are the exception to the rule,*** as Western civilization spread around the globe mostly without a religious motivation at the center of its motives - thanks to the Treaty of Westphalia, which, maybe not coincidentally, has come in for rough treatment recently from the alt-right. But it's probably not a coincidence that this vacuum was eventually filled with political philosophies. The ideas of democracy have been carried (imperfectly) at the head of the most successful empires of the day, and needed no marketing. But as the Rise of the Rest continues, the values of liberal democracy and reason may need some slick PR and catch phrases, otherwise we may regress to the historical mean. What would the beginning of Huntington's era of future civilizational religious struggle look like? One aspect would be a drop in the global status of liberal democracy, very similar to what we've observed in the last few years with the rise of China, Brexit, and the election of Trump.
**As an example of a 1990s-ism in this book: the Chechen conflict has a prominent place. Then again even the clearer-thinking Peter Turchin suffered from this myopia a bit when in the 1990s he predicted the rise of an Islamic Chechen state.
***Huntington does overstate the exceptionalism of Europe's nonreligiousness, which bears expanding in a footnote. For one thing, it's interesting that there could even be a Peace of Westphalia, and that a (second!) religious schism was tolerated in Europe. There is no such equivalent between Sunni and Shi'a who have coexisted in severe tension often erupting into war, almost since the death of Mohammed, and his thoughts on what aspects of Europe or its culture made coexistence possible would be useful. Second, Europeans certainly were partly motivated by extending Christendom (applying more to Catholic powers than Protestant, and more earlier than later) and an attempt to reclaim what Christians believed were rightly their own lands (the Crusades, and the Iberian Reconquista, both obvious civilizational conflicts, and the latter of which he gave no attention, even though there are Spain-Morocco tensions to this day.) Third, he rightly observes the bizarre coincidence that Europe didn't originate its own religion and today practices a Middle Eastern one, but leaves out the observation that Buddha was blue-eyed Indo-European whose religion spread to East Asia and is nearly absent from the land of his birth. Huntington somehow concludes by stating that Europe is the exception to history's rule for not originating a religion that it then evangelized to the rest of the world, but really there have been only three successful evangelical religions (Christianity, Buddhism and Islam), only one of which began and obtained political power in the region of its birth (Islam). It's actually Islam and the Islamic world which are the exception. As Islam can be thought of as Abrahamism v3.0 (Christianity is the earlier version), from an evolutionary standpoint we would expect it to have to be more virulent and power-seeking to surpass its antecedents. The obviousness of the replacement of religion in later Western expansion with political philosophies seems to need little comment.
Several of these analogies have been lucidly argued elsewhere, and I've provided relevant links. Whether history rhymes or repeats, these patterns may prove instructive to anticipating the near future.
1. The end of the Belle Epoque and the Eve of the First World War. We don't need more essays despairing the rise of nationalism in turning politics away from globalism, heralding the end of the Davos order, but the rational detente of late 1800s Europe fell apart quickly in the face of nationalism and entangling alliances. Trump's election by nationalists and his entanglements with Russia expose this similarity. Today, Russia increasingly tests whether people in Cleveland want to risk a nuclear war over Lithuania. Little appreciated today is the shock World War I represented for globalism (or at least European internationalism), resulting in a decrease in international trade that didn't return to pre-WWI levels until the 1970s. For me, this analogy is the most poignant of all that I list here, since there are clear cyclical waves trade and political relationships, some of them quite macro. The most obvious is the onset of the middle ages in Europe after the fall of Rome, but even this was a repeat (and a pale one) of the dark ages of antiquity, the late bronze age collapse in the Mediterranean. A decrease in international relationships is of obvious advantage to regional powers with chronically anemic economies but large militaries, like Russia.
2. The Washingtonian Dynasty Losing the Mandate of Heaven. It's a truism that Chinese dynasties last on the order of two centuries. This pattern holds for other large states. Taking the Roman Empire as a succession of two states punctuated by the Crisis of the Third Century, we have two dynasties of not quite two centuries. The U.S.'s founding moral authority and legitimacy stem from our belief in the specialness of our constitution. Though I count myself a patriot I must admit that if I heard an Australian or Mexican or Indian talking about her constitution with the same unquestioning reverence that we hold toward our own, it would seem rather strange. Red and blue state Americans have not begun to question the constitution in earnest as much as have irreconcilable ideas of what it means. If the American Civil War was nearly a North-South constitutional echo of the East-West scriptural schism of Europe, this may be the Reformation. What has suddenly exacerbated this difference? One possibility is a status monoculture brought about by social media. There are few things worse by being looked down upon by morally illegitimate people, pretending to be in the right based on the principles that only your own side understands.
3. The internet as the printing press and the American cultural divide as mid-millennium Europe's Protestant-Catholic divide. The internet and in in particular social networks have suddenly and inescapably forced on Americans the realization that there are people elsewhere in our own country with fundamentally different values than our own, and different ideas about the origins of our government's moral legitimacy. This is problematic, because humans really have only three ways of dealing with the "other": remain ignorant of them (which we no longer can), convert them, or decide that they are subhumans/gentiles/outlanders who are not worth converting (or to whom conversion can or should not apply). In mid-millennium central Europe, the printing press not only spread ideas but spread awareness of the people in the city next door who despise your moral authority and who might even try to force you to follow theirs. Even empires cannot comfortably or sustainably solve the disappearance of moral-authority silo walls; the Ottoman Empire had the unique solution of millets, but even this was uneasy and eventually collapsed. For a much more thorough treatment of aspects of the Thirty Years War analogy see Venkatesh Rao's essay.
4. The end of the Whig party and Trump as Zachary Taylor. Of course, part of this story must be the dramatic realignment of political coalitions occurring on both sides of the Atlantic, a shift similar to but more profound than the cultural-coalitional mismatch Nixon exploited in his Southern strategy. That realignment is the shift of cultural conservative blue collar whites into the GOP and the more surprising transformation of wealthy coastal professionals into the Democrats. It's absurd to assume that a country of 320 million can be adequately represented by two political parties, and surely both major parties have their contradictions, but the GOP's tripolar values of God-country-market is far more explosive. In practice, those three values reduce to two values - respect for authority, and individual freedom - because God and country largely covary. Respect for authority versus protection of individual freedom further correlate closely with intelligence and education, and the basic cognitive divide that modern economies and elections expose are vexing for traditional coalitions of conservatives and liberals alike. (Odd, that educated liberals fight for entitlement programs that in many places are going to uneducated people to collect while they sit at home watching FOX.)
The analogy here is between the GOP and the Whigs, an American national party that died in the 1850s with an outsider President, driven by internal conflict over a question that exposed their divisions. When slavery was was forced to the center of national attention by the admission of southerly states after the Mexican War, the Whig coalition (of Northern industry-men and Southern plantation owners) disintegrated - having until that time been held together by their hatred of over-reaching Federal executive Andrew Jackson. As we are learning, a party that knows its values only in opposition to a hated enemy can rarely sustain itself. Mexican War general Zachary Taylor was a complete outsider to politics who was elected as the last Whig President. It might be mentioned here that if you count up the governors and members of Congress who came to their position with no history in politics, you'll notice that Republicans dramatically outnumber Democrats. The President is only the most dramatic such example. For an expansion of this analogy see Gil Troy's essay.
5. Trump as Carter. I won't belabor this one except that a) you can’t do much better than this essay by Julia Azari and b) pushing the analog probably too far, that would make Obama the Democratic Nixon, rather than the Reagan. Aside from the paranoid streak that was Nixon's undoing, this would not otherwise be an insult: both were tireless public servants and serious policy wonks, with pragmatic centrist styles. (Nixon a centrist you say? He established the EPA and laws protecting whales, removed the U.S. from the gold standard, and proposed national health insurance, which may today be as unbelievable as Obama defending the TPP may appear in a few decades.)
Four of the five examples end in war. I hope that Peter Turchin's prediction of a maximally turbulent 2020 is about "mere" civil unrest and not civil war.