Caroline Elkins made front-page headlines a decade ago when her research into Britain’s brutal suppression of the Mau Mau movement in Kenya in the 1950s resulted in a high court case and, uniquely, reparations to 5,228 surviving Kenyans who, the British government accepted, had been subject to years of systematic torture and abuse. That case relied on evidence uncovered in Elkins’s 2005 book, Britain’s Gulag, which had argued that up to 320,000 Kenyan Kikuyu people had been held in British detention camps as part of a campaign of terror that “left tens of thousands, perhaps hundreds of thousands, dead” and untold numbers of lives ruined by forced labour, starvation, torture and rape.

When Elkins’s book came out, her findings – partly based on the testimony of Kikuyu survivors – were widely dismissed as, at best, exaggerations by a generation of historians wedded to stubborn ideas of Britain’s “enlightened” and “benign empire”. Her history was dramatically vindicated, however, when an unknown cache of 240,000 top secret colonial files, removed from Nairobi at the time of Kenyan independence in 1963, were disclosed on the eve of the 2011 trial. The files had been stored in a high security foreign office depository at Hanslope Park, near Northampton. At the time of that high court victory, Elkins noted that she had for years put on hold a wider inquiry into the methods of British colonial governance in the years after the second world war, in order to substantiate the survivors’ case, research that would now be illuminated by the fact that the secret document store also held “lost” records from 37 other former colonies. She was both vindicated and outraged by the discovery: “After all these years of being roasted over the coals, they’ve been sitting on the evidence? Are you frickin’ kidding me? This almost destroyed my career.”

This book, a decade on, is that wider history that Elkins had postponed. Partly resting on the Hanslope Park files, it argues that the sadistic methods that marked the last acts of empire in Kenya were not an anomalous aberration but learned behaviours of imperial power...