Theresa May did two significant things this week. The first, her decision to postpone Brexit, is the dominant story of the times. The second has received less attention. At prime minister’s questions, Mrs May prefaced her answers by talking about the “shameful scar on British Indian history” of the killings at Jallianwala Bagh in 1919. “We deeply regret what happened and the suffering caused,” said Mrs May. When Jeremy Corbyn replied, he went further, calling for a “full, clear and unequivocal apology for what took place”.

These brief exchanges were a reminder of the long shadow cast by what is better known in Britain as the Amritsar massacre, whose centenary falls on Saturday. It is rare for a Conservative prime minister to express regrets for any aspect of British imperial history. So the fact that Mrs May said anything at all was noteworthy: first, as a sign of continuing official unease at the highest level about the events of 1919 and, second, as a recognition of the effect the massacre still exerts on the British-Indian relationship to this day.

Mr Corbyn’s response was significant too. By using the word “apology” he went further than any British leader has yet been willing to go. When the Queen laid a wreath at Amritsar in 1997, she called the massacre “distressing”. When David Cameron visited Jallianwala Bagh in 2013, he wrote in the visitors’ book that the events were “deeply shameful”. The difference between Mrs May’s and Mr Corbyn’s words was important, for this is a long and an unquiet argument that is not yet resolved.

The events of 13 April 1919 were terrible and, to this day, are insufficiently known in Britain. Imperial troops (made up of Gurkhas, Baluchis and probably some Sikhs) fired without prior warning into a peaceful crowd of more than 15,000 for 10 minutes. An official commission put the total of dead at 379, with more than a thousand injured, but the true figures may be higher. The shootings caused outrage and anti-British radicalisation in India. The effects on Gandhi were decisive. What took place at Jallianwala Bagh was never forgotten. The lieutenant-governor of Punjab at the time of the killings was assassinated in London in belated revenge as late as 1940. The 1919 killings do not stand alone. Others preceded and followed them. But Amritsar remains to this day the most potent embodiment of the violence on which British rule partly rested for nearly two centuries.

Liberal Britain was scandalised by Amritsar too. Winston Churchill (then a Liberal) said it was “a monstrous event” and “a slaughter”. The India secretary Edwin Montagu called it simply “terrorism”. But there were widespread and lasting efforts to whitewash and ignore what happened. Many in the Raj and in Britain, not least within the most reactionary wing of the Conservative party, approved of it wholeheartedly. When General Reginald Dyer, who ordered the shootings, died in 1927 he was given a military funeral in Somerset, followed by a second, ceremonial funeral in which his coffin, draped in the union jack, was wheeled through central London on a gun carriage as if he was a national hero. Dyer still has his defenders today.

The reluctance to apologise has many strands. They include concerns about precedent, legal consequences and claims for reparations. But the reluctance to look back dispassionately, understandable in some respects, is a national burden. It means Britain can fail to face historical facts, question ourselves as a modern nation and think about complexity. It can mean we fail to see ourselves as others see us. These are enduring issues, which cannot be brushed aside just because they are sometimes exploited opportunistically.

Some other countries are better at this self-examination. Germany is one. Belgium, which is trying to rethink its own imperial past, is another. Britain can learn from them. Britain lacks a shared or a sufficiently capacious version of its own history. Too many are not taught enough of it to make this possible. Outside the academy walls, and sometimes within them, the treatment of history can be too politicised, nationalistic and manichean. The result is that we don’t think properly or even know about events like Amritsar. But the result is also Brexit.

• This article was amended on 23 April 2019 to add clarifying detail of the ethnicities and religions represented among the Imperial troops who opened fire at Jallianwala Bagh.