You could argue that the Civil War began on May 22, 1856, when Rep. Preston Brooks of South Carolina entered the Senate chamber and beat Sen. Charles Sumner of Massachusetts nearly to death with his cane.

Brooks’s casus belli was Sumner’s freshly delivered antislavery speech, in which he called Sen. Stephen A. Douglas of Illinois a “noise-some, squat, and nameless animal,” and suggested that Sen. Andrew Butler of South Carolina — Brooks’s cousin — was in love with “the harlot, Slavery.”

You could argue that Sumner was more than justified in using such uncivil language, given the enormity of the evil, slavery, that Douglas was willing to tolerate and Butler actively defended.

And you could argue that this bloody attack on the senator, if it catalyzed the war, only hastened the inevitable and necessary.

What’s perhaps most relevant today, however, is the fact that Brooks’s attack exposed beyond deniability the degree to which Americans of the 1850s lived in separate realities. Newspapers in the South not only justified what Brooks had done, they made him a hero; the Northern press did the same for Sumner.

Our broken polity has not yet produced one such climactic breakdown in civility, but we seem to be on our way there, and because so many people seem to relish that — the slogan “F--- civility” is gaining traction — it’s important to be as clear as possible about both the causes and the potential consequences.

Civility spreads, as a democratic norm, on the basis of consensus. When most people involved in politics share broad goals and a belief in the overall legitimacy of the political process, they are more likely to disagree, when they do, without being disagreeable.

This is more a matter of incentives than education and training: When democracy is about competing but not warring groups taking turns in power, it’s in your self-interest to be civil because doing so sets a useful precedent — for you.

When civility prevails among the politically active, it spills over into calm and normality for everyone else; where you shop, or dine, or go to the movies doesn’t have to be a political statement. Commercial establishments are, in that sense, safe spaces. Just ask any citizen of a former communist state in Europe what it was like when every trivial life choice carried political freight.

The broad political consensus that prevailed in the United States between the end of World War II and the end of the Cold War has broken down, and civility has broken down as well. This didn’t happen by accident: The first turns in our downward spiral occurred in the early 1990s, when Newt Gingrich broke all kinds of taboos and norms en route to securing a Republican House majority.

By now, however, there’s plenty of blame to go around. The dominant modus operandi of American politics, across the political spectrum, is to attack various elements of the postwar consensus as attributes not of a stable democracy, but of “a rigged system.” Aspirants for elected office compete to delegitimize not only each other but also American-style democracy itself.

Of course, consensus can mask corruption. A middle-of-the-road two-party system built on the exclusion of certain groups, based on their racial or other identity, lacks legitimacy, even if a lot of people in power say otherwise.

Certainly the division and chaos that led to the Brooks-Sumner clash, and the Civil War, had been a long time coming, notwithstanding Congress’s attempt to suppress it through various compromises and, for a time, a ban on even discussing — “agitating,” was the word — the “slavery question” in the House.

What we have now is the unprecedented situation of the president of the United States not only attacking the legitimacy of the American political system, not only trashing the very idea of consensus, but also agitating every single question of policy, no matter how sensitive, in the most inflammatory words possible. On basic factual questions, he repeatedly asserts that two plus two equals five, and many seem to believe him.

He is doing all of this from a position of partisan dominance, or at least advantage, over the federal government and most of the states as well.

Those whom this approach inevitably disempowers and threatens now reach for their own unconventional weapons — ostracism, heckling, mobbing. Rep. Maxine Waters (D-Calif.) has said of Trump’s Cabinet, “tell them they’re not welcome anymore, anywhere.”

You can understand why people act and feel this way without immunizing them against basic questions such as how far, exactly, they propose to go, and what, exactly, they hope to accomplish.

Michelle Obama once declared, “when they go low, we go high.” That might not work, either, but it does have one clear advantage: It is the only option that could, even at this late date, inspire an upward spiral.

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