This is how democracy died in Chile. Before the 1973 coup, Chile was Latin America’s oldest democracy, buttressed by vibrant democratic norms, including a well-established “culture of compromise.” Chileans liked to say that there was no political disagreement that could not be settled over a bottle of Chilean cabernet. But beginning in the 1960s, Chile’s culture of compromise was shattered by Cold War polarization. Mutual toleration eroded, and political parties eschewed forbearance for a “win at all cost” strategy. Chilean democracy fell into a death spiral, culminating in a bloody coup. (The intervention of the United States accelerated but did not cause this death spiral.)

Could it happen here? It already has. During the 1850s, polarization over slavery undermined America’s democratic norms. Southern Democrats viewed the antislavery position of the emerging Republican Party as an existential threat. They assailed Republicans as “traitors to the Constitution” and vowed to “never permit this federal government to pass into the traitorous hands of the Black Republican Party.”

Norm erosion alters the zone of acceptable political behavior. Partisan violence pervaded Congress. Joanne Freeman, a historian at Yale, counted more than 100 incidents of violence (including fistfights, canings and the pulling of knives and pistols) on the floor of Congress between 1830 and 1860. Before long, the republic would be broken — and Americans would be killing one another in the hundreds of thousands.