In an interview with Salena Zito for the Washington Examiner that aired on Sirius XM on Monday afternoon, President Trump expressed his continued admiration for President Andrew Jackson with a remark that struck many as surprising, to say the least.

“I mean, had Andrew Jackson been a little later you wouldn’t have had the Civil War. He was a very tough person, but he had a big heart. He was really angry that he saw what was happening with regard to the Civil War, he said, ‘There’s no reason for this,'” Trump said. “People don’t realize, you know, the Civil War, if you think about it, why? People don’t ask that question, but why was there the Civil War? Why could that one not have been worked out?”

On the one hand, though Andrew Jackson died in 1845 — nearly two decades before the Civil War began — Trump’s belief that Jackson would have prevented the war from happening, while impossible to prove, does have some connection to the real events that happened during Jackson’s presidency. Specifically, Jackson was President during the nullification crisis between South Carolina and the federal government over the question of tariffs. During that episode, South Carolina asserted the state’s right to void federal law, a preview of the Confederate view that those states could withdraw from the Union.

Jackson responded in 1832 with a proclamation that explained his view that for South Carolina to take up arms to back up that position would be treasonous, and Congress authorized him to use the military to enforce the tariff in question. As Jackson put it, “Our Federal Union—it must be preserved.”

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The federal government also, however, compromised on the tax to which South Carolina had objected. South Carolina backed off and no use of force proved necessary. Jackson, then, did express his anger over this particular matter that would come to a head in the Civil War, as Trump put it, and his complicated legacy does include getting credit for heading off an earlier armed conflict.

But the question of “why was there the Civil War” is one that people have in fact been thinking about for more than 150 years.

While a nullification dispute over taxes could be solved without war by the president and Congress, the dispute over slavery could not. And yes, even Andrew Jackson, in the 1830s, could see that coming. It wasn’t “worked out,” as Trump put it, because the South was too invested in the continued existence of slavery to let it be.

The difference between those two situations was explained by none other than Abraham Lincoln himself, in 1865, when he delivered his second inaugural address as the war continued to rage. He dedicated his extremely brief address to exactly that question. Here’s the portion of that speech that contains his answer, with emphasis added:

On the occasion corresponding to this four years ago all thoughts were anxiously directed to an impending civil war. All dreaded it, all sought to avert it. While the inaugural address was being delivered from this place, devoted altogether to saving the Union without war, insurgent agents were in the city seeking to destroy it without war — seeking to dissolve the Union and divide effects by negotiation. Both parties deprecated war, but one of them would make war rather than let the nation survive, and the other would accept war rather than let it perish, and the war came. One-eighth of the whole population were colored slaves, not distributed generally over the Union, but localized in the southern part of it. These slaves constituted a peculiar and powerful interest. All knew that this interest was somehow the cause of the war. To strengthen, perpetuate, and extend this interest was the object for which the insurgents would rend the Union even by war, while the Government claimed no right to do more than to restrict the territorial enlargement of it. Neither party expected for the war the magnitude or the duration which it has already attained. Neither anticipated that the cause of the conflict might cease with or even before the conflict itself should cease. Each looked for an easier triumph, and a result less fundamental and astounding. Both read the same Bible and pray to the same God, and each invokes His aid against the other. It may seem strange that any men should dare to ask a just God’s assistance in wringing their bread from the sweat of other men’s faces, but let us judge not, that we be not judged. The prayers of both could not be answered. That of neither has been answered fully. The Almighty has His own purposes. “Woe unto the world because of offenses; for it must needs be that offenses come, but woe to that man by whom the offense cometh.” If we shall suppose that American slavery is one of those offenses which, in the providence of God, must needs come, but which, having continued through His appointed time, He now wills to remove, and that He gives to both North and South this terrible war as the woe due to those by whom the offense came, shall we discern therein any departure from those divine attributes which the believers in a living God always ascribe to Him? Fondly do we hope, fervently do we pray, that this mighty scourge of war may speedily pass away. Yet, if God wills that it continue until all the wealth piled by the bondsman’s two hundred and fifty years of unrequited toil shall be sunk, and until every drop of blood drawn with the lash shall be paid by another drawn with the sword, as was said three thousand years ago, so still it must be said “the judgments of the Lord are true and righteous altogether.”

Though Lincoln had a personal stake in this analysis and many people have disagreed about it over the years, historians today generally agree that he was right from the beginning: the short answer is that slavery (of which Jackson was a defender) was why there was the Civil War.

Write to Lily Rothman at lily.rothman@time.com.