Near the beginning of the third century B.C., the Republic of Rome faced an acute threat to its domination of the Italian peninsula. In a series of brutal battles, Pyrrhus of Epirusand a fearsome parade of 20 war elephants had managed to vanquish Rome’s armies. When Pyrrhus offered Rome a comparatively lenient peace treaty, many of its senior statesmen were keen to take the deal.

It was, Edward J. Watts shows in “Mortal Republic,” thanks to the unrivaled strength of Rome’s political institutions that Pyrrhus’ victories ultimately issued in his proverbial defeat. When the Senate convened to debate the offer, “an old, blind senator named Appius Claudius was carried into the Senate house by his sons.” As the chamber fell silent, he stood to chastise his colleagues. “I have,” he said, “long thought of the unfortunate state of my eyes as an affliction, but now that I hear you debate shameful resolutions which would diminish the glory of Rome, I wish that I were not only blind but also deaf.” By giving in to Pyrrhus, Claudius warned, the Roman Republic would only invite more outside powers to mess with it. Low as the odds of victory might be, Rome had no choice but to keep fighting.

Unable to pacify the Roman Republic by treaty, Pyrrhus turned to bribery. When Fabricius, a senator widely known to be as poor as he was distinguished, arrived to negotiate a prisoner exchange, Pyrrhus offered him gold and silver so plentiful it would make him one of the world’s richest men. But Fabricius refused. “The Republic of Rome provides those who go into public life with everything they need,” he haughtily declared. Because even a poor man could accede to the most distinguished offices, his reputation was far more important to Fabricius than Pyrrhus’ money.

Taken together, Watts shows, these two speeches encapsulate the foundations of Rome’s remarkable success. At its inception, “the republic provided a legal and political structure that channeled the individual energies of Romans in ways that benefited the entire Roman commonwealth.” But over the following centuries, that foundation slowly weakened, and then rapidly collapsed. ...