On 10 February 1972 thousands of workers, acting in solidarity with the miners who were then on strike, surrounded the coke depot at Saltley in Birmingham.
The enormous mass picket stopped all traffic in and out until the bosses gave up and closed the gates of the depot.
As well as being the turning point of the strike, which the miners won, Saltley was a great symbol of what working-class solidarity could do.
Afterwards, when asked why he had not sent in the army to disperse the workers, Tory prime minister Edward Heath responded (so he wrote somewhere) with a question of his own: did the advocates of sending in the army, he asked, think that the troops should have had live ammunition in their guns?
If not, then they couldn't do the job. For Heath and those around him, it was unthinkable in Britain in 1972 for the army to shoot down mass-picketing workers.
But British troops with live ammunition in their rifles could be sent, and were sent, against peaceful demonstrators in the Irish - but UK - city of Derry. There, the British army officers felt themselves to be pitted against alien, rebellious "natives".
What happened in Derry may well have influenced Heath when it came to Saltley. There, on the afternoon of Sunday 30 January - one of a number of "Bloody Sundays" in modern Irish history, tne days before the working-class victory at Saltley Gates - the soldiers of the British Army Parachute Regiment were unleashed against unarmed demonstrators.
The people were demonstrating against internment - indefinite imprisonment without charge or trial - which had been introduced the previous August.
The troops fired at the unarmed demonstrators, and kept on firing until thirteen - many of them teenage boys - were dead, and another so badly wounded that he died soon afterwards. 17 demonstrators were wounded. At least one of the dead was killed going to the aid of the wounded.
According to the verdict of the Saville Inquiry, one of the soldiers killed four or five people. In plain words, he was a psychopathic killer, someone like the taxi driver Derrick Bird, operating with an Army licence to kill with impunity.
The marchers felt themselves to be the victims of a murderous, unprovoked assault. And they were. Simon Winchester, then the Guardian's correspondent in Northern Ireland, recently described his experience in Derry that afternoon.
"Then, incredibly, [the troops] starting firing, firing, firing in our direction. I was too stunned to wonder why: all I knew was that I had to get out of the lines of fire, and quickly.
"I ran and then, as bullets whizzed above me, dropped face down into a puddle of broken glass. A man fell beside me, blood gushing from his leg.
"I could see the soldiers taking up new firing positions, moving in a fan towards the crowd. I got up, raced toward a row of rubbish bins and dropped behind them, heart pounding. There was more firing. People were sobbing, cursing...
"A youngster of 16 or so was with me, terrified. At one point, the two of us managed to crawl on hands and knees up a slight rise, to a point below the city walls. I remain convinced that at this point a soldier fired at the two of us: I saw a soldier on the ground suddenly point his rifle at me, and his arms jerked, twice. I dived, and skittered up the laneway to a church..."
The soldiers of the Parachute Regiment were the British Army's equivalent of Rottweiler attack dogs. They had a deserved reputation for reckless brutality.
How they came to be loosed on the peaceful march in Derry was one of the mysteries of the whole affair. It is now revealed, four decades later, in the Saville report, that in giving them their head, their officer disobeyed orders.
After Bloody Sunday, the propaganda machinery of the British government went into overdrive. The British soldiers had been returning the fire of Provisional IRA men among the marchers, they said. Some of those who had been killed were armed IRA men. The blame was entirely theirs.
A government commission under Lord Widgery was quickly set up to establish what had happened. Widgery, a man of the Establishment, did what was expected of him. He concluded that the army's account was the truth.
Now the verdict of the Saville Inquiry, which sat in Derry for 12 years hearing testimony, brands Widgery and the British army liars.
The Derry massacre sent shock waves through Ireland. In Dublin, the British embassy was burned down.
The Belfast Home Rule parliament, which for 50 years had legitimised Protestant-sectarian government in Northern Ireland, was abolished two months later, and Britain assumed direct rule.
In Britain, Irish workers on a number of building sites struck in protest.
In London, the Saturday after Bloody Sunday, there was an enormous protest march in Whitehall which erupted into serious fighting with the police, including police cavalry. A sizeable part of the march consisted of previously non-political Irish immigrants.
I saw one group of six or seven young men take off their belts, wrap the leather round their hands with the buckles dangling, and wade into the police. That is what they had come for. So had an enormous number of others.
The military campaign of the Provisional IRA (and of the other IRA, the Stalinist-led "Officials"), was ten months old by Bloody Sunday. Though the Official IRA would go on a permanent ceasefire within a few months, support for the Provisional IRA increased enormously.
The introduction of internment in August 1971 - exclusively against Catholics, at first - had already thrown a large proportion of Northern Ireland's Catholics into the arms of the Republican militarists. "Bloody Sunday" redoubled the effect. The Provisionals would keep the support of about one-third of Northern Ireland's Catholics all through the subsequent 23-year war.
Workers' Fight, a predecessor of Solidarity, had not been wrong when it commented: "The 13 dead men shot down in cold blood on January 30th in Derry City will have as powerful a posthumous effect on Irish politics as did the 16 dead men killed in cold blood after the 1916 Rising". So they did.