A brave truth teller. That’s how the speaker identified himself, someone willing to defy political correctness to warn of a pressing threat to ordinary citizens. He claimed the nation was being overrun by foreigners who made white citizens feel like “strangers in their own country.” Whites, he argued, were becoming a “persecuted minority” because of laws that favored nonwhites.

Although the claims sound like those of Stephen K. Bannon, Richard Spencer or some other contemporary American white nationalist, the speaker was British Conservative Member of Parliament Enoch Powell. Fifty years ago today, Powell delivered one of the most influential speeches in 20th-century British history. He warned that nonwhite migrants from former British colonies were destroying his nation. If Parliament took no action to halt migration, Powell, a classics scholar, warned Britain would soon mirror Virgil’s dire vision of the “River Tiber foaming with much blood.”

It became known as the “rivers of blood” speech.

In Britain, the speech is well known and has generated much discussion in the buildup to its anniversary. It has particular salience in Britain today because children of Caribbean migrants with the right to remain indefinitely have been denied reentry into the country, thanks to government rules designed to create a “hostile environment” for immigrants.

But the speech’s relation to American history is far less known. “Rivers of blood” is part of a long history of transatlantic white nationalism. Those roots help explain the surprising 2016 victories of the Brexit referendum and Donald Trump, both of which relied on the notion that nonwhite immigration threatened their nation’s essential character.

In his speech, Powell famously quoted his Birmingham constituents, which allowed him to claim the mantle of populism, much as white nationalists do today. He recalled one who said he wished his children would settle overseas because “in this country in 15 or 20 years’ time the black man will have the whip-hand over the white man.” Another complained of a white woman tormented by the children of Caribbean migrants, whom she described as “wide-grinning pickaninnies.”

Political elites denounced Powell for inciting racial conflict, and he lost his place in the Conservative shadow cabinet. But his speech was widely popular, approved by 74 percent of British respondents in a Gallup poll.

Yet Powell wasn’t just speaking to the British. His speech relayed his “horror” at the racial violence occurring in American cities, where the rivers already ran red. In 1967, Powell had visited Detroit, scene of a destructive conflict between African Americans and police forces, and he kept closely apprised of further developments in the United States. He spoke just two weeks after the assassination of the Rev. Martin Luther King Jr. touched off a new wave of uprisings among urban African Americans.

To Powell, this violence was not the result of social injustice, economic inequality or police repression. Rather, it resulted from the existence of a sizable group of blacks who could not be assimilated into the American nation. Powell feared that mass migration of nonwhites to Britain would create a problem of “American proportions long before the end of the century.”

Powell’s message found a sympathetic audience among some Americans. In 1971, the Citizens’ Councils, the primary organization fighting desegregation in the South (sometimes known as the “country club KKK” because it combined its racism with middle-class respectability), invited Powell to speak at its headquarters in Jackson, Miss. On the political defensive in the wake of civil rights advances, Southern segregationists were eager to find allies abroad like Powell and the white supremacist governments of Rhodesia and South Africa.

Powell’s speech in Jackson, which alleged that “whites [in Britain] are being held back to accommodate the Asiatics and blacks,” met widespread approval among an audience of Citizens’ Council leaders and a host of local and state politicians.

Though his “rivers of blood” speech cost Powell his chance at political leadership, it remained a rallying cry for the British right for decades. The slogan “Enoch was right” often appeared at far-right rallies. And the person most responsible for advocating that Britain leave the European Union, Nigel Farage, declared in 2014 that he agreed with the “basic principle” of the “rivers of blood” speech.

Like Powell, the former leader of the United Kingdom Independence Party found admirers in Mississippi.

Fresh off the Brexit victory, Farage attended the Republican National Convention in July 2016. There he befriended delegates from Mississippi, who invited him to visit the state later that summer. Farage replied, “The idea of a trip to Mississippi? Rather. Absolutely.” Like Powell, Farage found a receptive audience in a state whose civil rights record indicates that both visits should be understood as part of a long history of exchanges among white nationalists in Britain and the United States.

Previous Republican presidential candidates might have avoided associating with Farage. But he fit well with Trump’s Republican Party. After all, two months earlier, arriving in Britain the day after the Brexit referendum passed, Trump had tweeted out his praise to Britons for taking “their country back.” (One wonders: Whose country was it, and who were they taking it back from?)

Farage had a long-standing relationship with Bannon, who had recently been appointed Trump’s campaign manager and who hailed the rise of right-wing European nationalism as executive chairman of the pro-alt-right website, Breitbart (whose London branch, which opened in 2014, advocated Brexit). Bannon introduced Farage to Trump in Mississippi, where the candidate was campaigning, and the two became fast allies. Following Trump’s election, Farage was the first foreign leader to meet with Trump, when the two took a memorable celebratory selfie before a glimmering set of golden elevator doors in Trump Tower.

The transatlantic crossings of Powell and Farage form part of a global history of white nationalism, one with particularly strong connections between the United States and Britain. Too many wrongly assume that because while nationalists are primarily concerned with the integrity of their nations that they are parochial in their politics. But global ties have long undergirded the pursuit of white racial nationalism.

Such connections, both real and imagined, have sustained white nationalists during times of apparent decline and inspired them at moments of triumph. The success of Brexit, for example, emboldened Trump’s nativist supporters to see his campaign as part of an international movement that could achieve power in the United States. We need to understand this history if we are to grasp why white nationalism remains such a force in our age of Brexit and Trump, and why Powell’s noxious ideas still resonate on both sides of the Atlantic.