National Interest

by Dan Edelstein

    One Ancient Roman historian argued that republics need the fear of common enemies to prevent political factions from tearing the state apart.

    The clarity of Thucydides’s writing and the acuity of his analyses led later readers to extrapolate from his History of the Peloponnesian War a timeless rule of international relations. Dubbed the “Thucydides Trap,” this rule holds that when a newer power (in his case, Athens) threatens the hegemony of an established power (Sparta), war becomes inevitable.

    Classicists and historians sometimes bristle at the way political scientists invoke Thucydides’ arguments. Might the identification of such a timeless rule make war more likely, say, between an emergent China and the dominant United States  At the same time, when politicians routinely deride the value of humanistic knowledge, it seems worth celebrating that an ancient historian can retain such lasting relevance. Parallels between classical times and our own are indeed fruitful, if not prophetic.

    Another similarity that has not escaped notice is the American Republic’s resemblance to its Roman precursor. Connections between the two have been drawn ever since the birth of the former. Our Senate, our Capitol Hill, our republican symbolism and Latin mottos—the American framers always kept an eye on a well-known and admired model.

Today, it is the waning days of the Roman Republic that attract attention. Is the United States on the verge of a new civil war  Will a charismatic leader declare himself a perpetual dictator, as Caesar had  Is it the fate of republics to dissolve into empires

    One can certainly point to any number of worrisome parallels. But we may also profit by returning to the histories of the late Roman Republic by contemporary writers. None of them has quite the same stature as Thucydides, nor have their works survived the ravages of time as well. But enough has been preserved for us to reconstruct what the best historical minds of Ancient Rome thought about the collapse of their own Republic—and how their findings might apply to ours.

• Discord, Avarice and Ambition: How the Roman Republic Fell

    The question of how and why the Roman Republic gave way to an autocratic regime preoccupied all the great historians, Greek and Roman, of their age. But one of them, Sallust, deserves particular recognition for blaming this collapse not merely on incidental factors, but on a general rule of history. A great admirer of Thucydides, Sallust is best known to posterity for his accounts of the Catilinarian Conspiracy (63 BC) and the Jugurthine War (111–105 BC). But it was in his Histories of Rome, which sadly exist only in fragmentary form today, that Sallust outlined a general political theory to explain the collapse of the Roman Republic.

    All classical historians had observed the cycle of civil violence that began with the Gracchi brothers, extended through the civil wars of Marius and Sulla and ultimately ripped the Republic apart in the time of Pompey and Caesar, whose adopted son Octavian rang its death knell as Augustus. But why was it only with the Gracchi that this cycle turned into a constitutional death spiral  After all, the early history of the Roman Republic was rife with civil disturbances. The plebs had seceded from Rome on multiple occasions and class strife led powerful generals such as Coriolanus to turn against his country. Tensions between the common people and the patricians were nothing new.

    Most accounts of why the Roman Republic fell focused on either constitutional or moral reasons. Polybius, a Greek hostage who spent much of his adult life in Rome and was a contemporary of the Gracchi brothers, predicted that the vaunted Roman constitution would eventually become unmoored, leading the people to choose a strongman to support and protect them against patrician predation. Cicero, whose own murder coincided with the death of the Republic, mounted a constitutional argument similar to that of Polybius, whom he greatly admired.

    Other historians opted for moral causes. In the Preface to his History, Livy, writing during the reign of Augustus, depicted a long and agonizing fall from moral probity: “as the standard of morality [disciplina] gradually lowers, let [the reader] follow the decay of the national character, observing how at first it slowly sinks, then slips downward more and more rapidly and finally begins to plunge into headlong ruin, until he reaches these days, in which we can bear neither our diseases nor their remedies.”

    Sallust’s own explanation falls into the moral category, but with a twist. Unlike Livy, but like Thucydides, Sallust took a dimmer, some would say realist, view of human nature. “The first conflicts among us arose from the vices of human nature, which is restless and untameable in its struggle for freedom, or glory, or power,” he affirms. The virtuous citizens of the early Roman Republic make no appearance in Sallust’s work. Human nature itself was defective. Little wonder that Sallust was St. Augustine’s favorite Roman historian (as we will see).

    But if violence and destruction are our fate, “as long as human nature remains the same,” as Thucydides maintained, why did the republic hold together for as long as it did  Livy’s narrative of a gradual moral decline and fall might seem better suited for explaining how the republic could last for nearly 500 years.

    But Sallust had a ready answer. Roman history was a long series of social conflicts, he insisted: “outrages had been committed by the most powerful men since the beginning, leading to disputes between the plebs and the nobles and other conflicts within the city.” Even Livy’s history did not disguise the near-constant clashes between social orders. The only times when Rome’s warring parts came together as a whole, Sallust maintained, were when an external threat loomed: “The state was governed according to equal and fair law after the expulsion of the kings, but only as long as the fear of attack by Tarquinius and the grievous war with Etruria still remained.”

    Once this fear receded, class conflict resumed unabated: “After that, the nobles began to rule the plebs like their slaves, making decisions about their lives and bodies in the manner of kings.” As with Thucydides (and later for Hobbes), fear was the motivating driver of human action.

    The ultimate collapse of the Roman Republic had no other cause. “Discord, avarice, ambition and all the other evils which commonly arise in prosperous times, increased greatly after the destruction of Carthage,” Sallust remarks. Here we find formulated what could be called, in homage to the Thucydides Trap, the Sallust Snare. The moment that the fear of foreign domination is removed once and for all, “prosperous times” pave the way to our demise. “When the fear from Carthage had been removed, they could resume their quarrels,” Sallust explains, referring to the social orders. The subsequent sequence of “regular riots, revolutions and eventually civil wars” was just Roman business as usual, this time without any external threat to play a mitigating factor.

    The reason why the fall of Carthage, as opposed to some earlier event, ended up precipitating the fall of the Roman Republic was that the Punic state was Rome’s last great contender for Mediterranean hegemony. As Sallust noted in the introduction of his Catiline, “when Carthage, the rival of Rome’s dominion, had perished root and branch and all seas and lands lay open, then Fortune became savage and threw all into confusion. Those who had easily endured toil, dangers, uncertain and difficult undertakings, found leisure and wealth, desirable under other circumstances, a burden and a curse.” This was how the Sallust Snare operated: what looked like prosperity turned out to be a poisoned chalice.

    The beauty and elegance of the snare is that no other causal factor is needed to explain the deterioration and ultimate collapse of the Roman Republic. This point is perhaps best elucidated through a comparison with other historians. In Livy’s narrative of moral decline, morals are either strengthened by institutions or corrupted by the lack of them. Education, religion, family customs and military discipline, in addition to legal and constitutional guardrails, are the necessary ingredients to sustain republican virtue over long periods of time.

    Sallust takes Ockham’s razor to this sophisticated republican theory. No amount of institutional engineering can prevent base human nature from corroding the state in the absence of a credible external threat. The human longing for security, which, in Hobbes’s later account, brings humans into the political state in the first place, was, for Sallust, the glue that held the state together over time. The centrifugal social forces that Thucydides, Aristotle and others had identified as the primary springs of conflict and revolution could only be harnessed by the real risk posed by an existential foe.

    Sallust also brushes aside the constitutional theories of Polybius or Cicero. What kept the Roman Republic from imploding, Polybius informed his Roman hosts, was the dynamic balance of its tripartite state (which Cicero later described as a “triplex rerum publicarum,” or “triple state”). The different functions of the consuls, senate and popular assemblies were so perfectly calibrated, he explained, that if one part “swelled up,” it would be knocked down to size by the other two. Hence, the constitution’s checks and balances—a phrase that we owe to translations of Polybius—would act as a correcting mechanism, preventing the state from lurching back into the Sisyphean cycle of regimes, or what Polybius famously dubbed the “politeion anacyclosis.”

    This style of constitutional analysis greatly impressed the American founders, who had been raised to see in the post-1689 English constitution a new and improved version of the Roman model. As an explanation for why the Roman Republic eventually failed, however, this analysis was not self-sufficient. Polybius therefore developed a theory of moral and civic virtue, supported by institutional measures, that was the necessary lubricant for his constitutional mechanics. Once the virtuous customs began to slip, the checks and balances would cease to function as planned and could not prevent a revolution that Polybius did not witness, but foretold.

If the course of events envisaged by Polybius is not far from what came to pass, what of the events that preceded  His own history of Rome began with the First Punic War, in 264 BC, roughly 250 years after the Republic was founded. Had civic virtue really flourished in the Republic throughout that entire earlier period  If not, can his constitutional cum moral explanation account for why Rome underwent regime change when it did

    The classical historian most convinced by Sallust’s theory was a writer whom we do not typically identify as a historian: St. Augustine. But The City of God begins with a detailed history of Rome, as part of an indictment of its fickle gods. How dare anyone blame the Goths’ sack of Rome on the Christian God, Augustine thundered, when Rome had suffered countless reversals while its inhabitants still worshipped pagan deities  He went on to list a long series of moral failings, starting with Romulus’s murder of his twin. Even in the early Republic, Brutus’s execution of his own sons could hardly be celebrated. The history of Rome was entirely “miserable.”

    Augustine agreed with Sallust that the Roman “lust for domination” (libido dominandi) was the source of all its ills. The brief spells when Rome enjoyed, as Sallust put it, “equal and fair law” occurred “not because the Commonwealth was devoted to justice, but only because she was hard pressed by her enemies.” Once “Rome’s rival for empire”—Sallust’s phrase—was destroyed once and for all, Augustine concurred, the Roman Republic was plagued “by a host of evils arising from the prosperity and security of her affairs.” The Christian theologian fully embraced Sallust’s historical lesson: “the sudden overthrow of Carthage is seen to have harmed more than did its prolonged enmity.”

    The Sallust Snare is thus a complement to the Thucydides Trap. If war is the likely result when a new power arises in the vicinity of an older one (much as Rome came into conflict with Carthage), revolution is the likely result when a power annihilates its last great rival. The Spartans did not avoid the trap, but they dodged the snare, refusing to sanction their allies’ proposal to destroy Athens after their victory. Both dangers revolved around the same emotion. It was fear that led the Spartans to go to war with the Athenians, just as the absence of fear led the Roman Republic to unravel.

No matter how persuasive, of course, Augustine’s testimony does not prove that Sallust was right. The empirical question of why the Roman Republic collapsed when it did is never likely to be answered once and for all. The payoff of reconstructing Sallust’s argument lies less in its felicity as a historical explanation than in its potential as a theoretical framework. How might Sallust’s Snare help us think differently about political instability and revolution

• The Cold War and the Punic War

    The most salient case that comes to mind concerns the United States after the end of the Cold War. For decades, two superpowers had lived in existential fear of one another. American politics during that time was much less polarized than at present and the US government accomplished many projects that are hard to imagine achieving today, from the interstate highway system to the creation of the best scientific research universities in the world.

    The collapse of the Soviet Union in 1991, however, arguably set into motion an internal American process of political destabilization. George W. Bush was the first president in 112 years to win office despite losing the popular vote. The Global War on Terror, launched after the September 11, 2001, attacks, briefly brought another existential foe into focus. But five years after American troops left Iraq, in December 2011, Donald Trump won the presidency, again without the popular vote. Upon returning in 2025, he has consistently pushed the limits of executive power, raising concerns about long-term harm to the US Constitution.

    Many commentators have focused on the person of Trump, viewing his combination of charisma and greed as a sui generis threat to the country. Others have pointed out that Trump’s actions—and his enduring popular support—mirror populist movements across the globe. Sallust provides us with a third lens through which to view Trump’s rise. In the absence of a common external threat, social forces tend to turn on each other. In Rome, these forces were primarily economic, with the wealthy optimates facing off against the poorer populares. In the United States, the social tensions are more cultural, pitting different demographic, geographical and religious groups against each other.

    What is the causal chain between these events  Sallust gave a simple, realist answer: humans have a “lust for domination” (libido dominandi) that resurfaces whenever fear of a common enemy subsides. It is not that very different from Francis Fukuyama’s argument in The End of History and the Last Man (1992) that, even though liberal democracy was left with no legitimate challengers after 1991, discontent with liberal democracy flared up all the same. Fukuyama drew on the Platonic notion of thymos, the “spirited” part of the soul that seeks distinction and shuns the kind of equality on which liberal democracy is premised.

    Plutarch had in fact already shown how, after the destruction of Carthage, the “love of honors,” or philotimía, thrust Roman leaders into a winner-takes-all competition that battered the republican constitution long before it fell. Marius accumulated an unconstitutional seven consulships; Sulla one-upped him by declaring himself dictator without a term limit. Trump regularly suggests he will ignore the Twenty-Second Amendment, which bars a president from serving more than twice in office.

    If Sallust is to be trusted, though, it is misguided to pin the blame for republican decay on a single person. Behind Tiberius Gracchus stood his brother; behind the Gracchi, Marius; behind Marius, Caesar; and behind Caesar, Augustus. Stopping any one of them did little to prevent the slide into autocracy.

    Sallust’s own theory offers little solace. In the absence of a credible external foe, what is one to do  Creating a common enemy carries its own authoritarian risks, transporting us to an Orwellian place where we have always been at war with Eurasia (or was it Eastasia ). But if the root of all social and political ills in prosperous times is our libido dominandi, then figuring out some means to channel this lust in less destructive ways may be our best hope.

    Machiavelli, who shared Sallust’s view that “when men are no longer obliged to fight from necessity, they fight from ambition,” noticed that the libido dominandi was strongest among the elite: the nobles “have a great desire to dominate,” whereas the people “have only the wish not to be dominated and consequently a greater desire to live in the enjoyment of liberty.” Designing a better system of distributive justice (ie, the conferral of honors and rewards) might help to manage elite ambitions by redirecting them toward civic ends. Congressional and judicial term limits could be one part of this effort.

• The Sallust Snare’s Cautionary Tale

    Hopefully, the Sallust Snare is not an iron law of political decline. For his part, Machiavelli insisted against Sallust that “those who blame the quarrels of the Senate and the people of Rome condemn that which was the very origin of liberty.” Starting from the same premise of vitiated human nature, Machiavelli reached an opposite conclusion, namely, that “all the laws that are favorable to liberty result from the opposition of these parties [popular and aristocratic] to each other.”

    Perhaps he is right and as Livy suggested, it is moral institutions, combined with sound constitutional structures, that preserve civic virtue in a republic. Justice Kennedy recently asserted as much in his 2025 memoir, Life, Law and Liberty: “no matter how brilliant the design of a constitution on paper, it cannot protect freedom unless there is goodwill, fair debate, moderation, restraint, reasoned decision-making and courage.”

    Still, the Sallust Snare, if not an inevitable trap, should serve as a cautionary tale. It is in times of peace and in the absence of existential threats that republics are most prone to internal conflict and collapse.