A quick (and true) parable from history: in 1189, Richard the Lion-Hearted decided that no Jews would be allowed at his coronation ceremony. When some leading London Jews showed up at the door, they were turned away, and when the gathered crowd saw this they concluded that the new King was solidly anti-Semitic and that the best way to celebrate would be to murder as many Jews as possible. Mobs killed almost sixty people and set the city's Jewish ghetto, the Jewry, on fire.

Of course, 12th-century London was mostly made of wood. It is impossible to burn only one neighborhood in a wooden city, and before morning a decent sized chunk of the city was on fire, too.

I think about that story from time to time, and more often lately, because it's a story about how uncontrollable civil violence becomes. You cannot burn one neighborhood and not the adjoining neighborhoods. You cannot start a fire and give it a list of people it should burn or not burn. Once it starts it is outside of your control. Political violence works the same way, through a political version of the same physics: once it starts it is difficult to stop. It spreads rapidly and unpredictably. It is in no one's control. It claims victims on every side, and innocent bystanders too. Everybody lives in a wooden city.

There has never been left-wing violence without right-wing violence in this country, never right-wing violence without left-wing violence. There was abolitionist violence as well as pro-slavery violence, anarchist violence and authoritarian violence, anti-civil-rights and pro-civil-rights violence. You can't read the history of Bleeding Kansas honestly and divide the killers from the martyrs along ideological lines. They go together. And once the violence begins, the violent make common cause against the rest of us, prolonging and intensifying the bloodletting as much as they can.

Am I saying that the violence was equal on both sides? No, and I am not the least bit interested in going through the box scores of old massacres. Am I positing moral equivalence for people on either side of these historical debates? No, because it's irrelevant. The fire doesn't care who's right. Am I ignoring who started the bloodshed in which case? Yes, I am, and so should you, because once the fire starts it's going to burn the just and unjust alike. The question is not who started it, but how to keep it from starting.

There is one civil peace, a single domestic tranquility, which protects us all. It is easy to disrupt and hard to restore. When it is disrupted, no one is safe. Every act of left-wing violence endangers people of the left. Every act of right-wing violence endangers people of the right. There is no safety but public safety.

The air in this country has been thick with inflammatory words since before the last election. It leaves an odor in the air, like gasoline soaking into rags. And when public figures speak of caution, some take that as partisan, or even as a provocation. That response strikes me as eerily disconnected from reality. The civil peace protects all equally, and if your political opponents want to preserve it, you should help them.

Still worse is keeping a selective list of partisan grievances, reciting a litany of all the horrible things the other side has done to your side lately while discounting the behavior of your own lunatic fringe. This accusatory stance can only hasten conflict, and never help to avoid it. And why does it matter if the "other side" has left more oily rags on the floor than your side? The question is how many oily rags pile up, not who does the piling, and you can only reduce the pile by reducing your own share of it. Throwing down more rags because "they" left even more is just self-destructive.

And discounting crimes against one's ideological opponents because the criminal was a lunatic or a loose cannon or not a "real" member of your movement is simply weak. The violent always come from the deranged and fanatical and weak-minded, especially during the build-up to a conflict. The fact that Abraham Lincoln didn't personally murder anybody in Kansas didn't calm anything down. Your side doesn't get to use the "just a nutjob" excuse because the other side's nutjobs won't honor it.

Progressive bloggers can discount the freak who bit off that tea-bagger's finger (!) and the freak who killed the poor demonstrator with the pro-life sign, claiming they "don't count," but there are people who are carrying around real or virtual press clippings of those events, building up their rage and justifying future acts of violence. They are counting those people. Conservative bloggers can claim that neo-Nazis like the one who shot up the Holocaust Museum "don't count" as conservatives, but the leftists most likely to commit atrocities count him. Every one of these people leaves another oily rag on our collective floor. Saying that we didn't put it there, and aren't responsible for removing it, is no help.

Civil violence is a lowest-common-denominator thing. The addled and hopeless are disproportionately attracted to it, and they are the primary audience for provocations. When a politician speaks in a way that reasonable people would only take as hyperbole or gamesmanship, that's not enough. What matters is how your speech is misunderstood.

Does it matter whether or not public figures intend to provoke violence? Well, to go back to my original story, Richard the Lion-Hearted never intended to start a pogrom. Of course not. He was an anti-Semite, but certainly didn't want any anti-Semitic bloodshed inside his kingdom. He was actually furious (he needed England's Jews to help finance his crusade), and did his best to stop the violence. But he could not. It spread to other towns and cities: to Norwich, to Lynn, to York. What Richard intended was not the point.

Dozens died in some towns. Hundreds died in York. It went on for months, well into the spring of 1190, like fire carried on a dry wind.