Jackson was prepared to use force, militia drilled in South Carolina, but a compromise averted the crisis. “We want no war, above all, no civil war, no family strife,” Henry Clay said in 1832, defending that compromise on the Senate floor. “We want to see no sacked cities, no desolated fields, no smoking ruins, no streams of American blood shed by American arms!”

But if Clay and Jackson averted war, their continuance was purchased in the blood and pain of others. There were 2 million slaves in 1830; by the time the Civil War came, there were more than 4 million held in bondage.

The question Trump says that “people don’t ask” may be the most debated historical question in America. Union veterans of the war tended to stress the moral imperative of their cause. But by the 1890s, historians like James Ford Rhodes were starting to understand the conflict as the inevitable clash of a slave system with an industrializing economy, with slavery less a moral cause than a tectonic force impelling the conflict forward. Charles and Mary Beard, in the 1920s, deemphasized slavery in favor of class conflict between the agrarian South and industrializing North, but saw the war as no less inevitable.

In the wake of the First World War, though, revisionists like James Randall and Avery Craven argued that the Civil War was a terrible, avoidable blunder. Slavery was inefficient, and left alone, would have extinguished itself, they insisted—it was weak politicians and crumbling institutions that produced unnecessary bloodshed.

This was the reigning interpretation when Trump was in school; if he studied the causes of the Civil War, it’s likely what he would have learned. And asking the question does Trump no discredit; when I taught the history of the Civil War to lecture halls of college students, we spent weeks discussing it. What’s alarming is the answer he proposes; that the conflict might have been averted by a strong leader. And the omission of a critical word: slavery.

By the civil-rights era, historical interpretations of the war were shifting. Historians looked more closely at slavery, and saw a rapidly expanding, even thriving, system. They looked at the words of those who pushed the nation into war. And they concluded that there was a remarkably straightforward answer to the question posed by the president: “Why was there a Civil War? Why could that one not have been worked out?”

Because the Civil War was fought over slavery. “Our position is thoroughly identified with the institution of slavery—the greatest material interest of the world,” Mississippi declared as it seceded. “The people of the slave holding States are bound together by the same necessity and determination to preserve African slavery,” said Louisiana. “The servitude of the African race, as existing in these States, is mutually beneficial to both bond and free, and is abundantly authorized and justified by the experience of mankind, and the revealed will of the Almighty Creator, as recognized by all Christian nations,” insisted Texas.