Sophy Roberts’ luminous new book is a journey through Africa from Zanzibar to Lake Tanganyika and back, retracing the steps of a long-forgotten expedition. Like her first, The Lost Pianos of Siberia, it explores a land and its history. Reflective, watchful, calm, Roberts is such a vivid travel writer that you forget what a brilliant historian she is. She has the water-diviner’s gift for stories in unlikely places. And then, through research in archives as well as on the ground, for uncovering sparky details that bring the story to life. Her conversations with everyone she meets, from hospitable nuns to a chief in a red fez, who pores over the original expedition maps with her at a Chinese restaurant in Dodoma, give her narrative a glow of sympathetic charm.

But she is telling a very grim story. The expedition she follows was dreamed up by King Leopold of Belgium, the plunderer of Central Africa at whose door the deaths of millions can be laid. Europeans at the time were preoccupied with how to get the plunder out: the French suggested flooding the Sahara. Leopold’s idea was to take Indian elephants and their handlers and use them to catch and train African elephants, then set up a supply line using these tsetse-fly resistant animals to transport Congo’s wealth out to waiting Belgian ships. Roberts’ recreation of their arduous route tells a tale “of blind ambition, violence and subjugation” which encapsulates the whole pitiless story of colonisation.

To carry out his plan, Leopold employed a charismatic Irishman, Frederick Carter. He made his name as a navigator on Mesopotamian rivers and spoke fluent Arabic, but knew little of Africa. Luckily, many of his letters and diaries survive and Roberts quotes from them so deftly that you feel you know him. She stays in places where he stayed, describing them as they are now and as he would have seen them. “As we retraced Carter’s steps, our views were his. Storks as slender as paint strokes picked their way along the bank.”

She begins her archival work, however, by researching Leopold in Brussels. Even his father thought him “sly”. Watching the British at work in India, Leopold realised what riches could be extracted from another people’s land. Publicly, he presented his plan for Africa as a way of getting rid of the Arab slave trade. In fact, his appointees cooperated with Arab slavers rather than removing them, and privately he confessed he was after “a share of the magnificent African cake”. After Carter’s expedition, decades of trade treaties and informal imperialism were replaced by a naked power grab: the brutal “scramble for Africa” between 1884 and 1914. Infrastructure built by Leopold led to suffering on an unimaginable scale and his vicious enforcers, says Roberts, “routinely chopped hands off the Congolese if rubber quotas were not met”. He was personally responsible, it is estimated, for the death of more than 10 million people.

It is Robert’s thoughtful reactions to these events in places where they happened, sometimes under the very same trees, that give her book its power. Carter, of course, failed in his mission. Of the four elephants he brought with him, all perished. Two died on the trek into the interior, where the party failed to find any wild ones to train. A third elephant died on the journey back and the last keeled over a week before Carter led his caravan into the Tanzanian town where bullets finally tore into his liver in a shootout.

Twenty years after his death, an elephant training centre was set up in Congo and was still going in the 1930s when Nadine Gordimer visited. It was the ghostly ruins of this school that Roberts came across in 2015, sparking her interest in the subject, and giving rise to a book whose ugly story is made bearable by her warm, beautiful writing, and equally warm human encounters.