I think Diane Abbott was wrong to malign all white people. They can't be blamed for all the bad things that happen to black folk or for our own bad behaviour. I wish though, that fire and fury hadn't burnt away the debates we could have had after her misguided tweet. Divide and rule was indeed the unofficial policy of the European empire, and the British were masters of that strategy. Post-colonial leaders learnt and used the same tactics. The gruesome Rwandan genocide happened because Belgian colonialists had institutionalised and encouraged tribal rivalries and Hutu leaders exploited the divisions.

One columnist suggested last week that Idi Amin's expulsion of us Asians was an example of divide and rule African-style. True. But my ancestors were set up by the British to be the racially exclusive middle class, a role we then relished. In the globalised world today, the West still plays that game, often working with loathsome dictators. Yet Britons don't care to be reminded of the policy and its effect on people.

Abbott believes in a "black community". There is no such monolithic entity but until the early Nineties, there was a united front of activists – Afro-Caribbean, African, Indian Pakistani, Bangladeshi, East African Asian, Chinese – all campaigning (with white anti-racist brethren) against racial injustice and discrimination. We were proud and "black". I told Justin Webb, the presenter on the BBC's Today programme, that I used to call myself "black" and he looked genuinely surprised.

What destroyed our unity was not white manipulation but our own internal divisions and bids for separatism. Abbott and I both miss that time when we had common cause and fight. This history is barely known today. It's gone, like so much else. Except for big war victories, the Holocaust, and unforgiven enemies – the Nazis, Japanese and now Muslim terrorists – key events and key players either disappear from memory or are spun into comforting fiction.

Paul Condon, now in the House of Lords, has been awfully quiet. He was head of the Met Police when Stephen Lawrence was killed. I went to some of the hearing held in the soulless Elephant and Castle shopping centre. Condon was contemptuous of allegations of racism in his force. Many years previously, he had been the deputy commissioner when a serious case of false imprisonment and malicious prosecution reached the headlines. A lovely Trinidadian, Frank Crichlow, ran a popular restaurant which was raided for drugs. He was eventually completely cleared and awarded compensation. And Condon did eventually promise to clean up the force, a promise half kept. Duwayne Brooks, who was with Stephen Lawrence when he died, claims that for years afterwards he was persecuted by the police and even falsely accused of rape. He cannot, of course, ever forget any of it. In 2006, the police paid him £100,000 compensation. I wonder if Lord Condon recollects any of the above.

When people have suffered atrociously, they rightly expect society to keep the candle burning. Richard Everitt, a white boy living near King's Cross, was murdered by a gang of Bangladeshi boys. I covered that killing and yes, I understand why his family and others think his death mattered too little. White people who are hated and attacked by non-whites are not lesser victims. Other black and Asian victims of racist violence are upset by the attention on Stephen Lawrence. I know his parents and what they have gone through – both have repeatedly said that racism affected others and still does. They remember; most choose not to.

Does the name Joy Gardner mean anything to you? A Jamaican mother, she was to be deported in 1993. Police and immigration officers turned up to take her and her five-year-old son. She resisted. The United Nations High Commissioner for Refugees report said: "Unauthorised equipment was used to restrain her in such a manner to lead to her death." Thirteen feet of tape, shackles, a leather belt and gag were used and the coroner concluded she died of brain damage caused by a lack of oxygen. Nobody was charged.

The poet Benjamin Zephaniah wrote a heart-turning poem about what happened. Not one person I talked to last week knew her story or about the continuing cruel treatment of migrants and asylum seekers and the sometimes lethal consequences that result. Absent-mindedness leads to apathy.

Maybe I remember too much and for too long, but millions of modern Britons barely remember yesterday, and with the internet culture the affliction is getting worse. An individual showing these symptoms would be diagnosed and treated. How does one treat a nation with such collective amnesia? When so much is expunged, no lessons are learnt. Little wonder the worst of history is so often repeated.

y.alibhai-brown@independent.co.uk