Fifty years ago today, on 20 April 1968, the austere shadow defence secretary Enoch Powell MP made a speech in Birmingham.
He told his friend, the editor of his local paper, the Wolverhampton Express and Star, that it would send a rocket into the air and light up the sky.
British politics has been cloaked in that rocket's shadow ever since.
Powell's River of Blood speech was a first and that's why it was so electric.
It was the first time that a major politician had spoken out against the cosy establishment consensus on immigration which had prevailed between both parties since the war.
By the late 1960s, hundreds of thousands of Commonwealth citizens had exercised their legal right and settled in Britain.
This was a hangover from the days of empire; to be a citizen of the empire in New Zealand or Canada or Kenya was to be indivisible from being a citizen of Britain. It was the original freedom of movement.
This had never been a significant problem. Transit costs were high and many of the colonies - especially the non-white ones - were poor. Few could afford the trip to the "mother country".
Slowly but surely, after the war, this began to change.
Moreover employers in Britain, facing a labour shortage, actively recruited in the wider English-speaking world, especially where workers were cheap.
Neither Labour nor the Tories had substantially different positions.
For various reasons, this settlement had held. By the 1960s, however, the then-Labour government had quietly begun to clamp down on numbers.
But no politician had spoken about it the way Powell had done. He said that in a decade or so "the black man will have the whip hand over the white man".
Powell, whose command of the English language was without compare, deliberately calibrated his words to shock and excite.
Thousands turned out on the streets to support him: "We want Enoch" rang from street corners across the land.
Finally, it was said, someone is speaking for us.
At the same time thousands more marched against Powell, and Commonwealth migrants were petrified. The nation was divided and has been ever since.
Some have argued that Powell was simply making a case about numbers; that the public services could not tolerate the strain of mass and concentrated immigration.
This is a partial interpretation. Powell does mention this but it is not the primary thrust of his argument.
Powell's greater concern was that immigration would erode the national character.
His greatest preoccupation is not even the immigrants coming but rather their descendants, the "native-born" who, he worried, would "constitute the majority" of the ethnic minority population in a few decades hence.
Britain, he said, "must be mad, literally mad, as a nation" to be allowing such "inflow". These native-born would be betwixt and between two worlds and fundamentally alter Britishness for the worse.
Powell's allies point to the wider context as mitigation. Powell had been an army officer in India (indeed, he adored the country).
He was there just before partition and had seen the stirrings of Hindu/Muslim violence for himself.
He had also just returned from the United States when he made the speech. Only two weeks before, Martin Luther King had been assassinated. America was contorted by race riots.
Powell was worried that the same was coming to Britain. He mentions the "American negro" a great deal throughout the speech and points out that in terms of the proportion of their respective minority populations, Britain was rapidly approaching American levels.
However, I think the more important context was more prosaic and substantially closer to home.
Powell was making the speech as the Race Relations bill was making its way through Parliament.
The bill would make it illegal to discriminate on the basis of colour or creed when engaging in commercial services.
In a world where the infamous "no dogs, no blacks, no Irish" was a commonplace refrain, this was a significant change.
The Conservative Party was unsure whether to support the legislation; and Powell's speech was designed to try and bounce Tory leader Ted Heath into opposing it.
Powell spoke of a letter from a constituent, a lady who had opened a boarding house but did not wish to let rooms to "coloureds".
"She is becoming afraid to go out," he said.
"Windows are broken. She finds excreta pushed through her letter box. When she goes to the shops, she is followed by children, charming, wide-grinning piccaninnies.
"They cannot speak English, but one word they know. 'Racialist,' they chant.
"When the new Race Relations Bill is passed, this woman is convinced she will go to prison. And is she so wrong? I begin to wonder."
There's been a lot of revisionism about Powell in recent years. Many argue that he was not a racist, that he would never think that whites were inherently superior to blacks or Asians.
I suspect that this is the case. However, such a definition sets the parameters of racism extremely narrowly. I suspect very few people would meet them.
But there are other ways of being a racist; using inflammatory language like "picanninies", arguing for a system where systemic discrimination on the grounds of race is allowed; arguing that national character would be eroded by native-born descendants of immigrants; if this doesn't meet the definition of racism, then what possibly does?
What value can the word even have?
Moreover Powell, for many years hence legitimised a certain type of nativism. Even today, the words "Enoch was right" can be found on many a far-right placard.
Here was a man who was learned, a scholar, fluent in Greek, well-heeled; but "even he thought it".
If anything, Rivers of Blood and Enoch Powell seem to me a case study not so much of racism but English attitudes towards class and academia.
Strip out the classical allusions - Virgil's River Tiber foaming with much blood - strip aside the clipped accent and the mellifluous tones and imagine what's left in a "working-class" voice; I'm not sure quite so many would be so reluctant to use the R word.
Powell was sacked by Ted Heath the day after he made the speech. He never returned to the front bench. Eventually he even left the Conservative Party, rather bizarrely reappearing in the House of Commons as an Ulster Unionist.
His career, as all political careers do, he once famously observed, had ended in failure.
But in another sense, Powell's politics did not end with him. Alongside Tony Benn, Powell probably has claim to be the most influential politician never to have become leader of the opposition or prime minister.
Alongside his thinking on immigration and the limitations of multiculturalism, he was the original eurosceptic.
He told my esteemed colleague Adam Boulton in 1993 that when we finally left the EU his name "would be on the roll of honour".
He was also a Thatcherite before Margaret Thatcher, extolling the virtues of monetarism and free markets long before they were fashionable and considered extremist.
Margaret Thatcher herself later acknowledged that her economic policy was largely his.
But we live in the age of Powell in a more substantial sense too. Powell's argument in Rivers of Blood is one which all of us find very familiar. He argued that he was not speaking for himself, but on behalf of his constituents.
Their concerns were being ignored by a powerful and poisonous elite. These elites were, without his constituents' consent, stealing their country from them.
He was the first modern British populist and his clothes have been stolen ever since.
As historian Robert Saunders of Oxford University told me: "If you take that speech in 1968, you have the manifesto of modern populism; the idea that speaking out against immigration is the act of a courageous and visionary statesman, it's the idea that what immigrants want is domination, what they want is the whip hand over the local population, it's the idea that what liberalism is about is about giving privileges and preferences to minority groups and that has been the position of populists ever since."
Powell was a man of enormous intellectual gifts, yet he was curiously oblivious to the ironies of that which he bequeathed us.
He had wanted a debate on immigration, but for years debate was stifled by the speech he made. He feared the division of a nation, yet who or what divided Britain more than Enoch Powell himself?