James K. Polk, the 11th president of the United States, is probably best known for growing the the size of the country by more than one-third. This territorial expansion pushed the U.S. border all the way to the West Coast, precipitating a heated national debate about whether to spread slavery to even more territories. Yet as white northerners in particular became increasingly uncomfortable with slavery’s expansion, Polk sought to downplay his personal investment in the institution.

Specifically, he concealed his purchase of enslaved children and young adults, whom he sent to work on his Mississippi cotton plantation while he lived in the White House.

Of the 19 enslaved people Polk bought during his presidential term (1845 to 1849), at least 13 were children, writes Lina Mann, a historian at The White House Historical Association. The youngest was a 10-year-old boy named Jerry. Polk kept his slave trading a secret by instructing surrogates to buy enslaved children and young adults on his behalf and then discreetly transfer them to him, according to Mann. He then sent them to work on his Mississippi plantation, which he purchased as part of the land rush that occurred after the 1830 Indian Removal Act violently expelled the Choctaw Nation and other Indigenous nations from their ancestral lands.

Polk’s secrecy wasn’t an attempt to cover up something he was ashamed of. Polk believed U.S. chattel slavery was morally correct, and there’s no indication he thought purchasing children—a cruel yet common practice during the slavery era—was wrong. It also wasn’t an attempt to cover up the fact that he owned slaves. This was a well-known fact when he ran for president on the Democratic Party ticket in 1844; and when he took office, he brought enslaved people with him to the White House. Polk was one of at least a dozen U.S. presidents to own enslaved people, eight of whom had served before him.

Instead, the reason for his secrecy likely had to do with changing opinions among white northerners about the morality of breaking up enslaved families. In an 1846 letter, Polk wrote that if the public found out about his purchases of children and young adults, “it would unnecessarily subject me to assaults from the abolition newspapers.” In addition, it would contradict some of the claims his campaign had made.

“By the time you get to James K. Polk, slaveowners are saying slavery’s actually this really great system because slaveowners really care about their slaves,” says Amy S. Greenberg, a history professor at Pennsylvania State University and author of Lady First: The World of First Lady Sarah Polk.

“It becomes a common thing for national politicians, if they own slaves, to say, ‘Well, I own slaves, but it’s only because I inherited them’; or ‘I own slaves because they’re part of my wife’s dowry, but I’d never buy or sell slaves unless it’s what the slaves want,’” she says. “And when Polk runs for president, this is what his surrogates on the campaign trail all do. They say, ‘Oh, James K. Polk has never bought or sold a slave except to keep families together.’”

If the public knew that Polk was purchasing enslaved children, they would know that this had never been true. “He described in a letter to his cousin [that] he preferred slaves who were ‘young and effective,’” says Michael David Cohen, a research professor at American University and former editor of the James K. Polk Project at the University of Tennessee, Knoxville.

“He wanted them, younger ones, because…they’d be around for a long time so he’d get his money’s worth out of them,” he says. “And also in the case of girls, they would have a lifetime over which they could give birth to additional slaves because any child born to someone he owned was considered his property as well.” Essentially, says Cohen, Polk viewed his Mississippi plantation and the enslaved people who worked on it as a retirement plan for him and his wife, Sarah Childress Polk.

Purchasing enslaved children was cheaper than buying enslaved adults, so Polk may have seen buying children as a way to increase his profit margin on his plantation. Greenberg notes that the prevalence of diseases like malaria in Mississippi, combined with the brutality of slavery, meant the mortality rate was quite high. About 46 percent of enslaved children in the antebellum South died before age 15; and on Polk’s plantation, that figure was “at least 51 percent, probably even higher,” writes historian William Dusinberre in Slavemaster President: The Double Career of James Polk.

Polk died a few months after his presidency ended, and his wife—who’d helped orchestrate his secret slave trading—continued to enslave the people he had purchased during his presidency. When the Civil War began two decades later, some Black men on the plantation escaped to join the Union Army. As soldiers, they fought to end the brutal system that their former owner—a president of the United States—had worked so hard to preserve, even as he had tried to conceal the true extent of his investment in it.