Children in pens. The overwhelming stink of human waste. Auctions at which human bodies were prodded, compared, and purchased. It was all in a day’s work at the New Orleans, Louisiana slave market, the largest and most lucrative in the pre-Civil War United States. But if it weren’t for a slave rebellion, Louisiana wouldn’t be part of the United States at all.

The Louisiana Purchase was one of history’s greatest bargains, a chance for the United States to buy what promised to be one of France’s largest and wealthiest territories and eliminate a European threat in the process. But the purchase was also fueled by a slave revolt in Haiti—and tragically, it ended up expanding slavery in the United States.

It would have seemed unthinkable for France to cede any of its colonial territories before 1791. The superpower had built a vast network of colonies in the Americas, capitalizing on European tastes for coffee, indigo and other commodities.

None of these held a candle, though, to sugar, which dominated French colonial holdings. And St. Domingue, which is now known as Haiti, was one of the great sugar capitals of the world. A full 40 percent of Britain and France’s sugar, and 60 percent of its coffee, was produced in Haiti, and the lucrative market lent itself to a particularly brutal slave trade.

A slave’s life in Haiti was usually short and miserable. So many enslaved people died of yellow fever and ill treatment that the entire enslaved population turned over every 20 years, and were held in subjugation through a strict caste system. Though there were 10 Black enslaved people for every white person in Haiti, the enslaved occupied the bottom rung of society and were treated like expendable commodities.

Meanwhile, Haiti’s free Black people were organizing. Inspired by republican ideals of liberty, fraternity and equality, they pressed for their rights, and some were given the right to vote in 1790 by the French government. But when the colonial government back in St. Domingue refused to recognize the law, the groundwork was laid for violence and revolt.

In 1791, the storm broke and thousands of slaves revolted. The revolution brought the colony to a state of insurrection and civil war. As slaves killed their enslavers and occupied and burned their plantations, white people defended themselves, then fled. The social order of the island crumbled and in an attempt to stop the violence, France abolished slavery. Under the leadership of Toussaint Louverture, slaves took over the entire island of Hispaniola, including St. Domingue and its neighbor Santo Domingo.

But Napoleon couldn’t abide the idea of the island being controlled by formerly enslaved people. Behind the scenes, he plotted to take the island back over and reinstitute slavery. But when French forces invaded Haiti in an attempt to restore the original order, the slave rebellion refused to budge. They burned cities, used guerrilla warfare and killed thousands.

France was in shock, and Napoleon began to realize that his dream of a French empire in the Americas might be doomed. He’d planned to send troops to Louisiana to take over the colony, which he had received from the Spanish through a secret treaty in 1800, in the hopes of using the territory as a trade venue for the commodities produced in Haiti. But if Haiti was under the control of the slaves, his plan was for naught.

Thomas Jefferson and his cabinet, themselves terrified of a French presence so close to the United States, used this conundrum as an opening. They approached the French with the offer to buy New Orleans, a port city of vital significance to American trade that they worried about France owning. To their surprise, France offered to sell them the entire territory of Louisiana instead.

As France and the United States negotiated the Louisiana purchase, Haiti became an independent country run by the victorious former slaves. But though the victory eliminated slavery in Haiti, slavery increased in the country that purchased the land Haiti had spooked France into selling. The Louisiana Purchase opened up a new can of worms in the United States—how much of the new territory should be open to slavery?

By doubling the size of the U.S., the purchase added vast swaths of territory that, pro-slavery advocates argued, should be filled with enslaved people. As farmers headed into the newly created Missouri territory with their enslaved, lawmakers tussled over the issue of which parts should have slavery. It took until 1820 for them to agree on the Missouri Compromise, which drew an imaginary line across the new territory that separated free and slave areas.

Slavery was now legal in Missouri, and the new state added pro-slave members to Congress. By 1860, there were more than 100,000 slaves in Missouri, and slaves were valued at over $44 million (about $112 billion today). Meanwhile, Louisiana, which also became a state after the purchase, remained a slave state, and New Orleans remained a critical hub of the slave trade.

So while a slave rebellion helped drive the Louisiana Purchase, the new territory was destined to become a place of suffering and exploitation for the thousands of slaves forced to work there.