A steady rain beats down on the steel roof of a rehearsal hall at the Massachusetts Museum of Contemporary Art, filling the cavernous space with a tinny thrum. Inside, The Head and the Load, a kaleidoscopic tableau of carefully controlled chaos, deftly steered by William Kentridge, has been going through its first-ever dry run; but the rain has brought things to a momentary halt.

“It appears we have some competition,” the South African artist says, wearily emerging on stage to address the small audience assembled for the piece’s run-through. “This is going to be slow, I’m afraid.”

Kentridge is accustomed to challenges, and over a decades-long career that’s produced an array of complex works ranging from drawing and painting to film and opera, no one could accuse him of thinking small. The Head and the Load, though, is something else entirely. Over just a couple of weeks here in the city of North Adams, the production has grown from snippets and fragments conceived in far-flung locales into a fully blown stage piece. It melds film projection, shadow play, dialogue in nearly a dozen languages, music, and a fluid, densely detailed set that seems to shift from moment to moment.

Whether it’s grown enough has been the question the artist can’t shake. “With an opera, we have six weeks to rehearse, and we begin with a score and a libretto,” Kentridge worries. “Here, we’ve had 15 days to make and rehearse.” The next time his cast of dozens assembles will be in London, five days before The Head and the Load opens in Tate Modern’s Turbine Hall. It’s the signature piece commissioned by cultural programme 14-18-Now to mark the centenary of the first world war, and Kentridge is feeling the pressure acutely.

“Typically, I’m awake at four in the morning with a panic of how to solve the questions of the day,” he says. “Here’s, it’s two till five.” Kentridge has taken to moonlight swims to cure his insomnia, finding other members of the production in similar straits. “One night William and I came across one another at half past one, in the pool,” said Thuthuka Sibisi, the production’s music director. “We worked up one of the scenes, solved some of its problems.”

The Head and the Load exhumes the largely hidden role played by African porters in the war’s spillover to the continent. In joining the war effort against Germany, hundreds of thousands of Africans were co-opted by French and British forces as load-bearers – four porters for each soldier – for the colonial forces. It was servitude motivated by practical-minded militarism – tsetse flies had decimated horses and oxen – and the hope, on the part of Africans, that their willingness to fight would lead to more equitable colonial rule in the ensuing peace.

Heavy load … ships were dismantled in east Africa and carried across the continent to be rebuilt. Photograph: Stella Olivier

That hope proved a vain one. At war’s end, France and Britain carved up Germany’s former African colonies between themselves, becoming the distant rulers of millions. Drawing up the Treaty of Versailles, the victorious powers sketched the outlines of what they described as the right to self-determination for all people.

It did not, however, apply to Africans; a delegation from the South African Native National Congress travelled to Versailles for the negotiations, but were refused a seat at the table. As a result, wrote the historian Richard Rathbone in The Journal of African History: “The war marked the period in which ‘pacification’ of both African and metropolitan critics of colonialism ends and colonial rule proper begins.”

“The confrontation of the war became such a crucible for heating up all the paradoxes of colonialism,” Kentridge says. “This, to me, is a way of putting those things together. This is not War Horse. It’s the idea of, ‘Are there different ways of telling a story?’ I think we’re trying to put beacons down: here is something to be excavated, here is the site of a mass grave we need to disinter – something real, more material, than the conclusion of a tribunal.”

Intense physicality … a porter carries a stricken comrade. Photograph: Stella Olivier

Indeed, The Head and the Load is marked by an intense physicality, from the epic sweep of its large-scale projections (vast historical maps of early-20th century Africa, carved into colonies, punctuated, in Kentridgean style, by blots and swipes of charcoal and ink) to the action on stage, where multiple scenes unfold simultaneously: the ebullience of an African cabaret, the quiver and jerk of an injured porter loping across the stage; and the show’s opening, a grand, chaotic procession of people and objects marching towards the field of battle.

Throughout, a swirl of languages — eight of them African, English, French, morse code in Hungarian and “fragments of bad German” — add to the enveloping haze, a kaleidoscopic swirl of action and sound that pushes back against the idea of history as a tidy, linear thing. “Among the many paradoxes of colonialism is this feeling of incomprehension,” Kentridge says. “Of language reaching a dead end.” On stage, the disconnects play out in both action and sound, underscoring the devolving calamity: too many voices, speaking at once, for meaning to be drawn from the din.

Within the disconnects, there is quantifiable disaster, and the numbers are telling, Kentridge notes. An estimated 30,000 soldiers died in the war in Africa — barely a footnote to its vast carnage, with 10 million military casualties in total. To that number, though, Kentridge adds the powerful context: 300,000 African porters and 700,000 civilians left dead as collateral damage.

Collateral damage … more than a million people died on the African continent in the first world war. Photograph: Stella Olivier

“In high school in Johannesburg, we would gather in the quadrangle, and the headmaster would read the names of the old boys who died in the first world war,” he says. “What was missing was all the other names. It’s very hard to find a list of names of African soldiers or porters who died. So in a way, this project is in part to give redress — to take note of that which we have chosen not to remember.”

The Head and the Load interrogates history itself, and intentionally so. “History is the victor’s tale, no?” Sibisi says. “That’s what makes this important, as much as anything else – the deshelving of history, who said what, and tacking this victor’s tale as the only version of things. What we have instead are these constantly competing perspectives. It’s a lot more true – and a hell of a lot more interesting, I think.”

Exhuming stories is tricky work, particularly now, as history unravels less as fact than perspective, with those typically shoved to the margins finding space closer to the centre. “On the one hand, yes, I think it’s important that black stories be told by black artists,” Sibisi says. “But I think certain stories can only find room through certain kinds of artists. This story, at this scale, happens because of William, and who he is — it becomes a global story, and that’s really important.”

‘I want more panic’ … William Kentridge at Tate Modern, London. Photograph: Sarah Lee/The Guardian

Indeed, Kentridge’s work, always focused on the gross inequities of the post-colonial world, have made him his country’s most visible artist. At the same time, he’s used his platform to elevate others: the performance laboratory he founded in Johannesburg in 2016, The Centre for the Less Good Idea, helps to incubate new works by an array of South African artists, all of whom benefit from Kentridge’s international profile. “William has been withdrawing from the idea of being South Africa’s foremost contemporary artist, and becoming more of an enabler of collaborative efforts,” Kerryn Greenberg, Tate’s curator of international art, tells me.

It’s also made him uniquely sensitive to the reckoning now at hand. “I understand people saying they need space to tell their stories,” he says. “That’s part of what the Less Good Idea is about. But for one artist to say to one another, ‘Stop working. I can’t breathe if you work,’ that my best contribution is to cut my hands off — I don’t agree. I understand it’s from a position of impurity, of compromise, of complicity within that structure. But that’s not to say there aren’t things to be done, works to be made, and different connections that are very important.” He sums up his thoughts. “I understand the complexities of the questions,” he says, “but I don’t understand the simplicities of the answers.”

On stage, the final rehearsal day ticks down from the morning to the afternoon, with Kentridge conferring with Sibisi, the actors and technicians almost moment to moment. MassMOCA’s director, Joseph Thompson, has invited the entire cast to his home for a pig roast at the end of the day, an invitation that seems to cause Kentridge more concern. “I don’t think we can afford to lose an hour,” he says.

One of Kentridge’s wearable sculptures for The Head and the Load. Photograph: Stella Olivier

On stage, he directs an actor in a complex choreography of swordplay, her giant shadow looming over a projected map of Africa as it’s carved, piece by piece, into colonies. “I want more panic, more uncertainty,” he says, as a cacophony of African languages erupts further downstage.

Today, indeed, will be long. Kentridge was hoping to end the piece with a collage of African independence movements, the direct fruit of the colonial powers’ oppressive seeds planted in the aftermath of the war. Now, he wonders if it’s possible.

“That feels like too long a second story,” he says. “But to give an intimation of the complexity at the end will both improve it sonically and as a narrative. Our ending is too pat. And the ending is never pat, is it?” True of both The Head and Load, and history itself.