You would be forgiven for not knowing which lesson, exactly, Americans ought to take from the bloody morning of September 13, 1859. On that day, in the mouth of a clearing by Lake Merced, in the hills of San Francisco, two men decided to settle an argument the old-fashioned way: with a pair of handcrafted .58-caliber pistols and a mutual death wish.

Theirs wasn’t the most famous duel in American history. But David Terry’s murder of his friend turned rival David Broderick that California morning is, I would argue, America’s second-most-famous duel, and possibly its most consequential …

I was thinking about Broderick and Terry recently after a gunman disguised as a police officer assassinated the lawmaker Melissa Hortman, along with her husband, Mark, in their Minnesota home last month. For many years I have been preoccupied by questions about political violence in America—most of all with the question of how to interrupt a cycle of political violence before more people are killed.