It’s not often that one can honestly say a book began as a speech. But my new book An Era of Darkness: the British Empire in India did just that.

At the end of May 2015, I was invited by the Oxford Union to speak on the proposition ‘Britain Owes Reparations to Her Former Colonies’. The event, in the Union’s impressive wood-panelled premises, was a success and I left pleased enough, but without giving the proceedings a second thought.

In early July, however, the union posted the debate on the web and sent me a video copy of my own speech. I promptly tweeted a link to it and watched in astonishment as it went viral.

Within hours it was being downloaded and replicated on hundreds of sites, sent out on WhatsApp and forwarded by email. One site swiftly crossed over three million views while others did not keep track, but reported record numbers of hits. Even the prime minister, Narendra Modi, congratulated me publicly for having said ‘the right things at the right place’. Hundreds of articles were written for and against what I had said. For months, I kept meeting strangers who came up to me in public places to praise my ‘Oxford speech’.

This is why my publishers persuaded me that the arguments outlined in my speech needed to be turned into a substantial book. It has just been published in India and is already the number one bestseller on several lists.

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Should a work of engaged amateur history have aroused so much passion? Seventy years after independence, shouldn’t we just forget about the past and move on? Is there still any moral urgency to explain to today’s Indians why colonialism was the horror it turned out to be? A lot of the popular histories of the British Empire in the last decade or two, by the likes of Niall Ferguson and Lawrence James, have painted colonialism in rosy colours, and this needed to be challenged. Historical material is available to everyone who’s willing to look for it, but perhaps, in the rush of modern materialism, we’ve stopped looking.

In three months’ time, the book will also be published in Britain, which has been suffering from a kind of historical amnesia about colonialism. As the book emerged from the press in India, an article by a Pakistani writer in The Guardian pointed out that the Brits simply don’t teach their own schoolchildren the truth about their colonial past. Many Brits are genuinely unaware of the atrocities committed by their ancestors and live in the blissful illusion that the Empire was some sort of benign boon to the ignorant natives.

There’s been a lot of self-justificatory mythologising in Britain about the colonial era. Popular television shows tend to focus only on the romanticised aspects of the Raj. All this explains Britons’ ignorance, but does not excuse it.

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British rule deindustrialised India, created landlessness and poverty, drained our country’s resources, exploited, enslaved, exiled and oppressed millions, sowed seeds of division and inter-communal hatred that led to the country’s partition into two hostile states, and was directly responsible for the deaths of 35 million people in unnecessary and mismanaged famines as well as of thousands in massacres and killings. That just skims the surface of the havoc wreaked by British colonialism. The British conquered one of the richest countries in the world and reduced it to one of the poorest. At the beginning of the 18th century, India accounted for 23 per cent of global GDP. When the British left it was down to barely 3 per cent. A country where landlessness and poverty were virtually unknown before the British, found itself at independence with 90 per cent of its population living below the poverty line.

Of course, many see lasting benefits from British rule. But each of these supposed benefits in turn – political unity, democracy and rule of law, the civil services, the railways, the English language, tea and even cricket – was designed to serve British interests and any benefit to Indians was either incidental or came despite the British.

But I don’t in fact ask for reparations, as the Oxford debate did. How do you place a monetary value on all that India suffered and lost under British rule? There’s really no compensation that would even begin to be adequate, or credible. The symbolic pound-a-year I’d suggested would be a nightmare to administer.

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Atonement is therefore the best we can hope for. An apology by the British would signal true atonement. Imagine a British prime minister, on the centenary of the notorious Jallianwala Bagh masssacre, apologising to the Indian people for that atrocity and by extension for all colonial injustices – that would be better than any sum of reparations. The British could also teach the harsh truth about colonialism to their schoolchildren instead of allowing them to wallow in romanticised ignorance about their own past misdeeds.

Yet the book is not intended to have any bearing on today’s Indo-British relationship. That is now between two sovereign and equal nations, not between an imperial overlord and oppressed subjects. Indeed, British Prime Minister Theresa May has just concluded a visit to India seeking investment from here in her post-Brexit economy. You don’t need to seek revenge upon history. History is its own revenge.

Former UN undersecretary-general Shashi Tharoor is currently a Member of Parliament in India