During the century and a half between the days of Pyrrhus and the rise of Tiberius Gracchus, there had not been a single outbreak of large-scale political violence. Then Tiberius pushed through land reforms in defiance of the Senate’s veto. In the ensuing fracas, he and hundreds of his followers were murdered. The taboo on naked power politics had been broken, never to recover.

Over the next years, it quickly became normal for populist politicians to set aside longstanding norms to accomplish their goals; for military commanders to bend the Senate to their will by threatening to occupy Rome; and for rival generals to wage war on one another. “Within a generation of the first political assassination in Rome, politicians had begun to arm their supporters and use the threat of violence to influence the votes of assemblies and the election of magistrates. Within two generations, Rome fell into civil war.”

If we are to avoid the fate that ultimately befell Rome, Watts cautions, it is “vital for all of us to understand how Rome’s republic worked, what it achieved and why, after nearly five centuries, its citizens ultimately turned away from it and toward the autocracy of Augustus.” In a sense, the book fails in this ambition. Especially as it progresses, Watts, a professor of history at the University of California at San Diego, abandons a careful analysis of the larger trends for a blow-by-blow account of the many conflicts that divided the republic in the last century of its existence. At times, this endless onslaught of calamities — a new violation of some traditional norm, the latest commander to threaten an invasion of Rome, one more shift in the ever-fragile constellation of power — starts to numb the mind.

But in another sense, the sheer repetitiveness of the calamities that befell Rome only serves to underline the book’s most urgent message. If we were to make explicit the implicit analogy that runs all the way through “Mortal Republic,” we would most likely cast Donald Trump as a farcical reincarnation of Tiberius Gracchus. Like the original populist, Trump was propelled to power by the all-too-real failures of a political system that is unable to curb growing inequality or to mobilize its most eminent citizens around a shared conception of the common good. And like Gracchus, Trump believes that, because he is acting in the name of the dispossessed, he is perfectly justified in shredding the Republic’s traditions.

If that analogy is right, the good news is that Trump will, once the history of our own mortal Republic is written, turn out to be a relatively minor character. Far from single-handedly destroying our political system, he is the transitional figure whose election demonstrates the extent to which the failings of our democracy are finally starting to take their toll.

The bad news is that the coming decades are unlikely to afford us many moments of calm and tranquillity. For though four generations stand between Tiberius Gracchus’ violent death and Augustus’ rapid ascent to plenipotentiary power, the intervening century was one of virtually incessant fear and chaos. If the central analogy that animates “Mortal Republic” is correct, the current challenge to America’s political system is likely to persist long after its present occupant has left the White House.