In this March 31, 2017 file photo, a portrait of former President Andrew Jackson hangs on the wall behind President Donald Trump, accompanied by Vice President Mike Pence, in the Oval Office at the White House in Washington. President Donald Trump made puzzling claims about Andrew Jackson and the Civil War in an interview, suggesting that he was uncertain about the origin of the conflict while claiming that Jackson was upset about the war that started more than a decade after his death. (AP Photo/Andrew Harnik, File)

NEW YORK (AP) — The U.S. president had a historical question: Why did America’s Civil War happen? “Why could that one not have been worked out?”

Remarks by Donald Trump, aired Monday, showed presidential uncertainty about the origin and necessity of the Civil War, a defining event in U.S. history with slavery at its core. Trump also declared that President Andrew Jackson was angry about “what was happening” with regard to the war, which started 16 years after his death, and could have stopped it if still in office.

Trump, who has at times shown a shaky grasp of U.S. history, questioned why issues couldn’t have been settled to prevent the war that followed the secession of 11 Southern states from the Union and brought death to more than 600,000 Americans, North and South.

“People don’t realize, you know, the Civil War, if you think about it, why?” Trump said in an interview with The Washington Examiner that also aired on Sirius XM radio. “People don’t ask that question, but why was there the Civil War? Why could that one not have been worked out?”

In fact, the causes of the Civil War are frequently discussed, from middle school classrooms to university lecture halls and in countless books. Immigrants seeking to become naturalized are sometimes asked to name a cause of the war in their citizenship tests .

Fierce disagreement over the future of slavery was a driving force behind the war, but economic issues and disputes over state rights were also factors.

“Slavery was the root cause of the Civil War. It was not the only cause, but it was the underlying cause,” said Eric Foner, a Columbia University history professor and a leading expert on the war. “As a historian, I would prefer the president had a better handle on American history.”

Trump’s comments about the war came after he lauded Jackson, the populist president whom he and his staff have cited as a role model. He suggested that if Jackson had been president “a little later, you wouldn’t have had the Civil War.”

“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 continued.

Jackson died in 1845. The Civil War began in 1861.

Late Monday, after a day of incredulous news coverage, Trump took to Twitter to amplify his message and seemingly stress that he did in fact know when Jackson died, writing: “President Andrew Jackson, who died 16 years before the Civil War started, saw it coming and was angry. Would never have let it happen!”

Jackson was a slave-holding plantation owner. Some historians do credit him with preserving the full Union when South Carolina threatened to secede in the 1830s over an individual state’s ability to void federal tariffs. But that controversy, known as the “Nullification Crisis,” was not about slavery, and the eventual compromise that preserved states’ rights did little to alter the nation’s path to the War Between the States.

“Even Andrew Jackson, were he alive, could not have solved the problem,” Foner said. “The situation in 1861 was far more dire than in the 1830s during the Nullification Crisis.”

The Civil War was decades in the making, stemming from disputes between the North and South about slavery and whether the union or the individual states had more power. The question over the expansion of slavery into new Western territories simmered for decades and Southern leaders threatened secession if anti-slavery candidate Abraham Lincoln was elected in 1860.

After Lincoln won without carrying a single Southern state, Southern leaders believed their rights were imperiled and seceded, forming the Confederate States of America. War erupted soon afterward as the North fought to keep the nation together. The conflict lasted four years.

The White House did not respond to requests for an explanation of Trump’s reasoning. His comments on the Civil War drew swift criticism from some civil rights groups and Democrats, including Rep. Barbara Lee of California who tweeted “President Trump doesn’t understand the Civil War. It’s because my ancestors and millions of others were enslaved.”

This is far from the first time that Trump expressed a muddled view on American history.

Trump, during an African-American history month event, seemed to imply that the 19th century abolitionist Frederick Douglass was still alive. Trump said in February that Douglass “is an example of somebody who’s done an amazing job and is getting recognized more and more, I notice.”

While justifying his argument for a border wall with Mexico, Trump said last week that human trafficking is “a problem that’s probably worse than any time in the history of this world,” a claim that seemed to omit the African slave trade.

Trump, prompted by his chief strategist Steve Bannon, embraced the legacy of Jackson soon after his election.

The White House has eagerly drawn parallels between the two men, particularly between Trump’s success with working-class voters and how Jackson fashioned himself as a champion of the common man against a political system that favored the rich and powerful. Trump paid tribute to Jackson, known as “Old Hickory,” by visiting Jackson’s grave in Tennessee in March.