Slaves For Peanuts is a story about a crop and the people who grew it at the margins of an empire. The slaves in this case were not in the southeastern United States, but in West Africa—specifically, in the humid, inland pastures which, in the 1840s, were the most abundant source of the world’s most important oilseed. Like many (if not most) imperial forays of the day, the peanut’s importance came down to a coincidence of taste, industry, and geography: Europe needed oil—to cook and grease machines, but especially to manufacture soap with. Olive oil was well suited to the purpose, but olive trees were vulnerable to frost and tended to grow in areas prone to conflict, making the supply unreliable. Palm oil made perfectly good soap and was popular in England, but its yellow hue was unappealing to French consumers. In the 1840s, French industrialists turned to peanut oil—and soon realized it could be used in a variety of ways, from cooking to fueling lamps.

In France, many consumers—already accustomed to white olive oil bars—likely didn’t even notice they were buying something new when soap manufacturers switched to peanut oil. But in Africa, the shift was nothing short of transformational. At the start of the peanut boom, France’s presence in Africa was limited to a few outposts on the westernmost edge of the continent: “more a hodgepodge of settlements than a cohesive colony, with an ever-rotating population of temporary agents for trading companies.” The largest, on an island called Saint Louis at the juncture of the Senegal River and the Atlantic coast, was a gateway through which the region’s wealth passed on its way to Europe. The rest of Africa lay just beyond it: “a whole continent—one where the French were guests, not hosts,” as foreboding as it was vast, and where practices which were illegal in French territory—including slavery—were widespread. While the French occasionally played the role of white interlocutor, the Africans of the interior were content to keep them at arm’s length.

Peanuts changed all of that. After France banned slavery in all its colonies in 1848, French colonialists began to see themselves as being part of a “civilizing”—not just a mercantilist—cause in Africa. But rising demand for peanuts had spurred demand for farm labor—and thus for slaves. As the French followed their commercial interests ever deeper into the African countryside, freeing more and more people from bondage as they went, the contradiction between their dual missions became harder to ignore. “The trading period has begun; it is said that there will be many peanuts this year,” one commander reported to the local governor in the late 1870s, eyeing the inevitable shift towards French control. “There is, however, one small dark cloud. It is the upcoming liberation of the slaves.”

The geopolitical game Lewis describes in Slaves for Peanuts is an old one, and one essential to the formation of the modern world: one side declares itself “modern,” “civilized,” or otherwise “advanced”—not just technologically, but morally as well—all the while depending on an influx of goods at a cost that’s only possible in the “heathen,” “barbaric,” or “underdeveloped” areas outside its control. As Lewis shows, division and denial allowed European soap buyers to stake a position of ethical supremacy without having to pay a great deal for the high standard of living they craved.

Readers will recognize parallel arrangements that make the comforts of our own world possible: think of how cheap agricultural goods like avocados (and workers) pass across the US-Mexican border, or how retailers in the US, Europe, Japan, and other countries source goods from China—where labor and environmental regulations are lacking—all while crowing about the “green” commitments they’ve made at home. For all of France’s proclamations of liberté, égalité, and fraternité, slavery was essential to both the moral and material architecture of French imperialism in Africa. But legitimizing moral hypocrisy has always been essential to making capitalism work.

One thing that has changed is our perception of those travesties. Modern technology makes it possible for rich nations to exploit poorer ones from a distance, allowing a degree of psychological dissonance. But in the 19th century, exploiting Africa’s lands—and the human hands that worked them—still required maintaining a nearby territorial presence. At least a few proponents of France’s “civilizing mission” had to live in close contact with the people who suffered under it, making the hypocrisy of the whole thing all but impossible to deny.

Lewis sifted the details of 19th-century Senegal mostly from yellowed letters, account books, and dispatches from archives in six countries. That alone is an astonishing achievement. What is more remarkable is that she was able to depict not just the early colonizers, but the Africans as well, including a few former slaves who either fled for French domains like Saint Louis—and rose to prominence in the tumultuous atmosphere of the colonies—or started their own communities nearby, like Kerbala.

Nonetheless, most of Slaves for Peanuts is necessarily limited to the stories of people whose lives history managed to record—namely, the missionaries and inland power brokers who dealt and corresponded with the French regularly. At times, the plot can be hard to follow, as their priorities shift and new people cycle in from Europe. The slaves are a constant presence, but they typically exist in the background, recognizable as a collective more than as individuals.

Where the story becomes most vivid, however, is in Lewis’s descriptions of landscapes, which she often renders more clearly than the characters who populated them. One can see how areas separated by the hard lines of colonial decree were brought together by the bonds of human connection. “Saint Louis prospered despite all the odds,” she writes, introducing the emergent destination for the liberated. She goes on: “It went from a sandy island with a small, fortified post to a proper city as traders and dealers imported stone from the Canary Islands to build houses, and people from the kingdoms up the river or across the dunes staked their tents or built houses of mud and reeds. Eventually, the city filled the whole island and had to expand. … Soon, bridges from island to island were built, to link the major points of the archipelago and provide for communication and commerce with the people up the river and across the dunes. Still, going to Saint Louis took fortitude and determination.”

Ultimately, depending on forced labor for such a basic commodity became untenable for a power that considered itself the vanguard of civilization in Africa. By the 1880s, after having indirectly encouraged slavery for decades, France (along with several other European powers of the day) declared an ambitious plan for conquering the rest of the continent and cited slavery’s persistence as a justification for it—marking the start of what the British called the “Scramble for Africa.” Hundreds of miles from the coast, at the groundbreaking for a new garrison in Bamako, Mali—one of the last stops of a future rail line which would connect West Africa’s interior to its coast and sustain French dominance in the region for generations—one French colonel told the crowd that slavery was “an integral part” of African morality, and one that Europeans alone had the responsibility to end. Having already spent hundreds of millions of francs attempting to abolish the practice, he said, “Republican France can spend a few million to modify, little by little, with wisdom and prudence, the vicious, unproductive, immoral system which is so beloved of all these peoples.”

By then, whatever memory French colonialists retained of having encouraged that “immoral system” had already faded. In time, the memory of the slaves themselves would pass as well.