On July 22, the day after a failed attempt at another such attack, one innocent civilian was killed by the police in what has turned out to be the hideously mistaken belief that he was involved in that failed attempt. Which of those incidents would you say constituted the greatest threat to the country at large? My guess is that you will believe the first event — the indiscriminate attack on ordinary people (the worst terrorist atrocity committed on mainland Britain in modern history) to be the one most urgently in need of analysis. For an instant, it got its due. For a brief moment the attention of the country focused on that horror: the agony of the victims’ families and then the shock of discovering that the act had been committed by people born and raised in this country.

Then came the shooting of Jean Charles de Menezes. A tragedy by any standards, his death was a direct consequence of the febrile atmosphere that followed the second terrorist plot that held, as a consequence of its failure, the prospect that four determined killers were still active in the city.

Now it is that death that consumes columns of print and hours of broadcasting debate to the extent that, as some of the July 7 victims’ relatives have noted, the attention of the country has almost completely shifted away from the premeditated murder that struck their families. So why is that? How is it that one wrongful death committed mistakenly by our security forces can obliterate coverage of the greater number of deaths committed intentionally by people openly dedicated to destroying our society? I got the first hint of what was to happen with the de Menezes shooting when amid the flowers on the street memorial to him, there appeared a printed notice likening his death to that of “all the innocent Iraqis who had been killed by the US and Britain”. Ah, I thought, here we go.

The tireless anti-war, anti-American, anti-government operation is moving in. Sure enough, within days we had the entire panoply of a classic operation by the sort of left-wing activists among whom I spent my youth.

The police had not just made a horrific error: they were cast as “executioners” on behalf of a government that was “racist” and determined to persecute anybody who looked vaguely foreign. And further, the confusion and obvious panic that led to misleading official statements after the shooting were part of a sinister cover-up: a conscious plan to deceive the country rather than a shambolic attempt to come to terms with an appalling misjudgment.

I scarcely needed to be told now that Yasmin Khan, a chief spokeswoman for the Justice4Jean campaign, was connected to the Radical Activist Network, which is opposed to “corporate globalisation” (the new name for what we used to call “corporate capitalism”). Or that another of its spokesmen was a veteran of George Galloway’s election campaign. How familiar all this was.

How much of the 1960s had I spent being summoned to support comrades who were striking at this factory or that town hall, or “organising resistance” to some council policy; to help to fill out and politicise a small crowd of protesters, or distribute leaflets that would “awaken the consciousness” of susceptible people involved in some dispute? On the New Left, we were experts at instant organisation: there was no conflict that could not be orchestrated for political advantage.

And the object was always to show that the capitalist government — and especially its police force — were the true oppressors. We were sent off to demonstrations with instructions to provoke the police into as violent a response as possible — in the presence of as many cameras as possible. The Grosvenor Square demo against the Vietnam war was a spectacularly successful instance of this.

Trotskyist newspapers such as the Black Dwarf (for which I wrote) and Socialist Worker — whose mass produced, professionally printed posters are still proudly carried at almost every public protest about anything — all made great use of those photographic spreads even though there were ideological divides between them. The Dwarf was regarded as corrupted and bourgeois by the organisation that produced Socialist Worker, then called International Socialism, because it was distributed at commercial outlets rather than sold by members on the street. The mainstream press, not wanting to be left out of the zeitgeist, joined in.

The implicit message is always the same: it is your capitalist (imperialist, racist, whatever) society that is the true enemy of the people. The supposed outside enemy — then it was Soviet communism, now it is Islamist terrorism — is just a pretext for the repression and victimisation of the people (or “working-class people” then), which is the real objective of the “ruling class”.

The groups in which I moved came in from the cold margins of political life when the Labour party, under James Callaghan, relaxed the rules on “entryism”, so that activists who had ties with openly Trotskyist organisations could become party members and thus Labour councillors. The result was the Militant Tendency — and the illustrious exploits of Lambeth and Liverpool councils, which put Labour in the wilderness nationally for a political generation.

Now in the world of Blairite Labour when all party politics has become moderate, the activists must once more channel their energies into specific causes. The Marxist dream having collapsed, the left has had to find a new raison d’être and in the absence of violent industrial conflicts, its need to expose the “fascist repression” of our police state can take a calamitous error like this and exploit it for all it is worth.

I’m sorry to have to put it as cynically as that: the genuine grief of the de Menezes family and the clear need to examine how the police could have made such an apparent mess of its own strategy, both obviously require urgent attention. But neither the grief, nor the police mistake, deserve to become fodder for those who have always wanted to promote social and political divisiveness — especially at a time when we need more than ever to be united.

Minette Marrin is away