At some point in the early summer of 54 BC, the Roman statesman Cicero set to work on his most consequential work of political philosophy: De Re publica (On the Republic). This exploration of what the Roman Republic had become, and what it was supposed to be, looked backward and forward in Roman history—and continues to have important implications for anyone living in a republic today.

Cicero set De Re publica in the year 129 BC, a dramatic moment when Romans, for the first time in centuries, had begun to confront the consequences of political violence. In 133 BC, a mob had killed the tribune Tiberius Gracchus after he used a combination of threats and extra constitutional measures to push through a series of land reforms. Four years later, the damage from Tiberius’s recklessness had become clear, but Rome still had a chance to mitigate it. So Cicero chose this moment to stage a dialogue in which the age’s most prominent politicians, jurists, and thinkers debated the nature of an ideal constitution and questioned what would become of their Republic after “the death of Tiberius Gracchus had divided one people into two factions.”

Cicero emphasized that Tiberius’s tactics of intimidation and his willingness to disregard law set Rome on a very dangerous course. “If this habit of lawlessness begins to spread,” he explained, it “changes our rule from one of justice to one of force.” This made Cicero “anxious for our descendants and for the permanence of our Republic.”

Cicero’s fear grew in part out of what he believed the Republic to be: community property of Romans, who were bound together not by race or ethnicity but by a shared sense of justice and fidelity to law. Law, Cicero wrote, provided the foundations for just interaction between citizens. It established the channels through which political decisions passed. And, because Rome was a representative democracy in which citizens elected leaders and voted on the legislation they proposed, Cicero argued that the Roman Republic could last forever if it remained governed by law and administered vigorously by its citizens.

A state governed by violence had much dimmer prospects. At best, such a state might sometimes “seem as if it was at peace” because “men feared each other … but no one was confident enough in his own strength” to challenge his adversaries. A sort of stable anarchy emerged, and a balance of fear was the only thing that held back citizen violence. Such a polity was no longer governed by laws. It could not be considered a republic.

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The United States now approaches the tipping point between a republic governed by law and the polity of violence, governed by mutual fear, that Cicero described over two millennia ago. Political violence courses through our streets as groups of demonstrators fight each other. The president and his political allies express public support for a young man who shot two protestors. It might be tempting to applaud Americans we agree with when they attack those with whom we do not, as Cicero did. But the distance from Cicero’s defense of Milo’s vigilantism to Caesar’s appeal to his troops is quite short. Neither vigilante justice nor armed insurrection can exist in a Republic whose citizens share a common purpose and respect for laws.

Cicero himself said so in one of the very last things he ever wrote before soldiers loyal to Mark Antony and the future emperor Augustus killed him, hanging his hand and head on the speaker’s platform in the Roman Forum. “Nothing,” Cicero proclaimed, “is more destructive to civilizations, nothing is so contrary to law and justice … than governing through violen