‘Are We Rome?” asked the author Cullen Murphy a decade ago, in a provocatively titled book that compared the 21st-century United States with the final days of the Roman Empire. Lately there’s been no shortage of Cassandras who emphatically answer yes.

“The speed with which we’re recapitulating the decline and fall of Rome is impressive,” opined the conservative commentator Bill Kristol, in a tweet this past July, taking issue with President Trump’s treatment of the press. “What took Rome centuries we’re achieving in months.” And the Cornell historian Barry Strauss bemoaned the parallels between alleged American failings and those of ancients in an online column for Fox News in October 2016: “Just think about what Rome had and what we have. Morals out of a bacchanalia? Check. Abuse of public office for private purposes? Check … Oh the times, oh the customs! Where is Cicero when we need him?”

Such comparisons are almost invariably political — pundits projecting onto the Roman story the particular modern grievances that most trouble them. But in the new book “The Fate of Rome,” Kyle Harper suggests that they overlook the big picture. Sure, the empire had internal problems — vicious leaders here and there, political coups, the disproportionate influence of moneyed elites, and trouble maintaining its far-flung borders. But Harper, a historian at the University of Oklahoma, has assembled compelling evidence that Rome died mainly from natural causes: pandemic diseases and a temperamental climate. If Americans want to compare our country’s faults to those of Rome, they might more closely scrutinize our biological and environmental vulnerabilities than our institutions alone.

Throughout the book, Harper positions nature as one character in a centuries-long drama. At the empire’s peak, the human actors — the political, cultural, economic, and military leaders who set up its institutions — were more than equal to the task. Under Marcus Aurelius, emperor from A.D. 161 to 180, about a quarter of humanity lived under Roman rules and influence. The Roman population swelled, wages rose, cities flowered (at its peak, the city of Rome had perhaps a million inhabitants), and vast trade networks threaded across Africa and into Asia.

But at the time, it was easy for Rome to make successful moves: Nature dealt it an especially good hand. During much of the Roman Climate Optimum (about 250 B.C. to A.D. 150), the empire was blessed with stable weather, abundant rain, and warm temperatures. Romans grew and shipped prodigious quantities of grain, especially in North Africa, and their leaders sometimes went to great lengths to hold wheat prices down, offer subsidies, and make sure citizens could feed themselves.

Then, from the middle of the second century onward, nature began dealing out some rotten hands — in the form of natural disasters and vicious germs — and the empire couldn’t hold its winning streak.

The germs were the most violent and obvious destabilizing forces. For all of the society’s technological sophistication, Roman doctors had no notion of germ theory, and Roman cities hosted a robust resident population of waterborne and airborne diseases —especially malaria, typhoid, and various intestinal ills.

On top of this, the empire’s densely urbanized populations — connected by intricate trade routes — were excellent targets for major pandemics. Harper demonstrates that the Roman Empire was hit by at least three great plagues, each a powerful blow to both its population and civic institutions. During one wave of the second-century Antonine plague, which was likely a form of smallpox, as many as 2,000 people died every day. A century later, a disease that sounds, from accounts written during that era, a lot like hemorrhagic fever (the gruesome Ebola family of diseases) migrated from Ethiopia across the rest of the empire and took a similar toll.

Meanwhile, the climate grew more and more erratic. “In winter there is not such an abundance of rains to nourish the seeds,” wrote Cyprian, an early Christian writer of Carthage. “The summer sun burns less bright over the fields of grain. The temperance of spring is no longer for rejoicing, and the ripening fruit does not hang from autumn trees.”

Drought struck the empire’s breadbasket of North Africa. The combination sent the society reeling, but it was able to recover until the climate swung again. In the fourth century, when the Eurasian steppe also fell under drought, nomadic peoples like the Visigoths and Huns (whom Harper describes as “armed climate refugees on horseback”) began to antagonize and terrorize Roman territories in Europe. Famously, the Visigoth leader Alaric sacked Rome in 410, effectively sounding the death knell of the Western part of the Roman Empire, which eventually fragmented into small, feudal territories.

The empire’s smaller eastern version, a.k.a. the Byzantine Empire, fared relatively well through the early years of Justinian, emperor from 527 to 565. Justinian — who came from a family of commoners and married a former actress and prostitute — led the empire with a style about as brash and contentious as today’s politicians. But he was also a man of action. “He was loved and loathed,” Harper writes, as he clashed with political elites, altered the tax code, and launched a massive program of government-sponsored engineering projects, with a special focus on flood control and aqueduct repair.

But by 541, the third and most gruesome of Rome’s germ invaders arrived in the Mediterranean — the Black Death, bubonic plague. The disease broke out as many as eight times between 542 and 747, and raged across the empire, carried by fleas riding on the bodies of rats. Wherever plague arrived, it killed about half to 60 percent of the population. As if that weren’t enough, the Mediterranean region suffered through a period of floods, unproductive cold summers, and dismal frigid winters now known as the Late Antique Little Ice Age. It’s hard to imagine any civic institutions that could withstand that kind of mortality and disaster.

Historians have long neglected the role of these actors — germs and climate — in bringing down Rome, partly because the evidence was simply unavailable. “Most histories of Rome’s fall have been built on the giant, tacit assumption that the environment was a stable, inert backdrop to the story,” writes Harper.

Some of the most interesting passages in “The Fate of Rome” describe how historians are now partnering with researchers in genetics, archaeology, and climatology to open up whole new avenues of inquiry into the past. Advances in DNA sequencing have made it possible to verify the presence of bubonic plague in ancient graves, for instance, and more definitively document some of its whereabouts in antiquity. Meanwhile, new research in historical climatology has painted more detailed pictures of Roman-era climates.

In the end, if Harper is correct, the story of Rome and its climate and germs is more deterministic than hopeful. American society does resemble Rome, but in even more alarming ways than we previously understood. Though modern health care systems give us a formidable advantage in the fight against disease, our hyperconnected globalized society is still an excellent medium for transmitting powerful germs. The next H1N1 or Ebola outbreak could have a far more calamitous outcome than those of the recent past.

Meanwhile, we’re at the precipice of a rapidly destabilizing climate, this time thrown off kilter by the same energy sources — fossil fuels — that helped create our prosperous modern global institutions. The two phenomena are connected, since the warming climate may be expanding the range of some diseases, including Lyme, malaria, Zika, and West Nile virus. Still, we know far more about both the causes of climate change and the ecology of germs than our ancient ancestors did. Perhaps we have a fighting chance of avoiding Rome’s fate, if we heed the true lessons of its fall.

Madeline Ostrander is a freelance science journalist based in Seattle. She has written on climate change for Undark, and her work has appeared in The New Yorker, Audubon, and The Nation, among other publications.