The Windrush story was not a rosy one even before the ship arrived

This is a year so overflowing with anniversaries that it was perhaps always going to draw our attention to the histories of race and migration in Britain. June marks the 70th anniversary of the arrival of the Empire Windrush at Tilbury Docks, carrying 492 West Indians who were looking to rewrite their fortunes in a Britain desperate for labour. The Windrush is now so much part of British history that almost instantly it became the shorthand used to describe the generation of black Britons whose plight has so shocked the country.

Friday 20 April was an anniversary of a darker kind, 50 years since Enoch Powell delivered his “rivers of blood” speech. That toxic diatribe, with its unsubtle references to “piccaninnies” and the “whip hand”, remains politically radioactive half a century later, as Radio 4 discovered last weekend when it broadcast the speech in an anniversary documentary. Today is the sombre anniversary of the murder of Stephen Lawrence, 25 years ago.

Who are the Windrush generation?

They are people who arrived in the UK after the second world war from Caribbean countries at the invitation of the British government. The first group arrived on the ship MV Empire Windrush in June 1948.

What is happening to them?

An estimated 50,000 people face the risk of deportation if they never formalised their residency status and do not have the required documentation to prove it.

Why is this happening now?

It stems from a policy, set out by Theresa May when she was home secretary, to make the UK 'a really hostile environment for illegal immigrants'. It requires employers, NHS staff, private landlords and other bodies to demand evidence of people’s citizenship or immigration status.

Why do they not have the correct paperwork and status?

Some children, often travelling on their parents’ passports, were never formally naturalised and many moved to the UK before the countries in which they were born became independent, so they assumed they were British. In some cases, they did not apply for passports. The Home Office did not keep a record of people entering the country and granted leave to remain, which was conferred on anyone living continuously in the country since before 1 January 1973.

What is the government doing to resolve the problem?

On Monday, the home secretary Amber Rudd announced the creation of a new Home Office team dedicated to ensuring that Commonwealth-born long-term UK residents would no longer find themselves classified as being in the UK illegally.

But there is another 2018 anniversary that, until last week, might well had passed by quietly, hardly noticed. This year marks 70 years since the passing of the 1948 British Nationality Act, which was being debated while the Windrush was crossing the Atlantic; gaining royal ascent in July 1948, as the Windrush pioneers were settling into their new jobs. Although now obscure, it was a law that Powell once referred to as “that most evil statute”. Much of what has happened over the past week can be traced back to that forgotten but critical piece of legislation.

The act was intended to reaffirm what many in the late 1940s regarded as a “time-honoured principle”, the doctrine that all British subjects should have the automatic right to travel to and settle in the United Kingdom.

But that noble and inclusive principle had been established in earlier decades, in the secure knowledge that poverty and the high cost of international travel would keep poor black and brown people far from Britain’s shores. Only a tiny minority – sailors, students, the very wealthiest and the most adventurous – would ever have the funds or the opportunities to come to Britain; fewer still would seek to settle here.

The people whom the architects of the 1948 act imagined would make use of their time-honoured rights were white citizens of Canada, South Africa, Australia and New Zealand. People of “British stock”, whose homelands in the “old commonwealth” were sometimes tellingly referred to as the “white dominions”.

Then, as now, official thinking had the unfortunate habit of falling behind economic and technological change. By the late 1940s, the cost of international travel had collapsed and a generation of men and women had emerged who had travelled vast distances during wartime and acquired a far greater understanding of the empire of which they were part and the opportunities that it might offer them in peacetime. Even before the 1948 act had reaffirmed their pre-existing rights of travel and residence, they began to arrive in Britain.

Part of the incredulity and outrage that has erupted at the cruelty shown to now elderly members of the Windrush generation stems from the way we have come to remember the Windrush moment. In one telling of the story, it has become a feelgood part of modern British history. The Pathé newsreel footage of the ship’s arrival conspires to that effect. The breathless commentary is unrelentingly upbeat, portraying the Windrush pioneers as loyal British subjects who had served the nation during war – as many of those on board had – and had returned in peacetime to perform yet another service for the Mother Country. When the Trinidadian calypso singer Lord Kitchener (Aldwyn Roberts) emerged on the deck of the Windrush, he was prompted, by the men from Pathé, to perform a few lines of his newly composed song, London Is the Place to Be.

The impression is of an organised and officially sanctioned programme of immigration. We know that those arriving faced appalling discrimination, but we also want to believe that in the end it all worked out and we became the inclusive, diverse nation that we like to believe we are. Rather, as the story of abolition has come to obscure the story of the two centuries of slavery that preceded it, an overly rosy version of the Windrush story risks standing in the way of us taking a real hard look at the politics of race and migration in the period in which the Windrush generation arrived.

Even before the Windrush had left Jamaica, the prime minister, Clement Attlee, had examined the possibility of preventing its embarkation or diverting the ship and the migrants on board to East Africa. After the vessel had arrived at Tilbury, the colonial secretary, Arthur Creech Jones, is said to have reassured his cabinet colleagues that, although “these people have British passports and must be allowed to land there’s nothing to worry about because they won’t last one winter in England” (detailed in Randall Hansen’s book Citizenship and Immigration in Post-War Britain).

When that prediction was proved false, ministers began to consider how they might revoke the commitments enshrined in the 1948 act. What followed was a two decade-long political struggle to change Britain’s immigration law and reduce the flow of immigrants from the so-called New Commonwealth. This is the other side of the Windrush story. In 1971, a new immigration act finally achieved that aim and stemmed the flow of migrants from the New Commonwealth. The same law granted those who had already arrived indefinite leave to remain.

That would have been the end of the story, had not, in 2013, those thousands been pushed into Theresa May’s “hostile environment”. The current crisis is a relic left by the political struggle to row back from the commitments made in the 1948 act.

Yet even, as now seems probable, the Windrush generation are evacuated from the “hostile environment”, that system, which has now been publicly demonstrated to be Orwellian in its vindictiveness and indifference, will remain in place. Other groups, for whom less sympathy exists, or is likely to develop, will remain its targets.

• David Olusoga is a historian and broadcaster