Last week, the United States Congress made an extraordinary intervention in British politics.

The "Friends of Ireland" Caucus, which includes both Republicans and Democrats, made it clear they would not support any US-UK trade deal if Britain's exit from the European Union in any way jeopardised the Good Friday Agreement.

It's not surprising the co-chair of the caucus, Democrat Richard Neal, would say such a thing. But Mr Neal was supported in his statements by his Republican colleagues.

In the current political environment, such a bipartisan consensus over potentially defying the will of President Donald Trump is noteworthy, to say the very least.

Given just how much political stock both Mr Trump and British Prime Minister Boris Johnson have put into the potential for a US-UK trade deal, this congressional threat has serious implications.

But it also gives us an unusual insight into how white supremacy works in the US.

Northern Ireland and the Republic have enjoyed an open border since 1998. ( ABC News: Lincoln Rothall )

A threat to peace

Brexit, and the potential it creates for a hard border between the Republic of Ireland and Northern Ireland, is a direct threat to that peace.

The 54-member Friends of Ireland Caucus is arguing that the US is obligated to defend the peace in Ireland, and they are right to insist the Good Friday Agreement be preserved.

Under the presidency of Bill Clinton, the US played a crucial role in facilitating the negotiations that led to the 1998 Good Friday Agreement. That agreement marked the end of three decades of sectarian violence in Ireland and the UK. The US has continued to act as guarantor of the peace.

But that doesn't entirely explain Republican willingness to defy Mr Trump.

Support for the Good Friday Agreement within the United States suggests that, despite a lot of evidence to the contrary, long historical memories still exist in politics. The Clinton administration played such an important role in the historic peace negotiations precisely because of a long historical connection between Ireland and the United States — one that has never been forgotten.

Irish connections run deep

In the US, there was always considerable support for Irish republicanism because of that connection — one that is built, at least in part, on resentment of the British. The United States was born, of course, out of rebellion against British rule.

And from the early 1800s, Irish people migrated to America in huge numbers to escape harsh conditions created and maintained by British rule. In the century between about 1820 and 1930, an estimated 4.5 million Irish people arrived on US shores.

In the US, the Irish connection runs deep. So deep, in fact, that every year on St Patrick's Day the Chicago River runs emerald green.

There is much to be admired and celebrated in this long historical connection. But it also has a sinister side — one that may play an important role in the future of British and trans-Atlantic politics, and one that we would do well to be wary of.

Journeymen Plumbers dye the Chicago River green to celebrate the start of St Patrick's Day celebrations in Chicago. ( Jeff Haynes: Reuters )

There's more to the love of Ireland

When the President of the United States encouraged the idea that American congresswomen should be "sent back to where they came from," his supporters came out to defend him with a vengeance.

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White House adviser Kellyanne Conway, after being questioned about the President's racism, demanded to know a journalist's ethnicity. Forced to defend herself for asking such an outrageous question, Conway insisted her point was merely to note: "We are all from somewhere else 'originally'," and that she herself had Italian and Irish heritage that she was "proud" of.

It's this invocation of Irish heritage that hints at what else is going on when Trump supporters profess their love for Ireland.

Last year, prominent right-wing media commentator and Trump supporter Bill O'Reilly visited the Emerald Isle to connect with his own heritage. While he was there, O'Reilly tweeted: "Enjoying my time in Ireland. Visited County Cavan where my ancestors were evicted from their land in 1845. That forced them to come to America legally so they wouldn't starve. Pardon me if I reject the 'white privilege' scenario if applied to my family."

Setting aside O'Reilly's egregious use of the term "legally", his tweet revealed a great deal. For many Americans, connection to Ireland is not just about family history and a shared resentment of British repression — it's also about shared whiteness.

This understanding of whiteness is based on the insistence that racism does not exist because, as O'Reilly claimed, white people are oppressed too.

In some of the darkest corners of the internet, this idea even goes so far as to include the belief that Irish people were actually enslaved, and suffered as much as, if not more than, African-Americans and people of colour.

Just like the other ideas that inform white supremacist ideology, this is a persistent and dangerous myth. As the events of last weekend prove once again, that danger is far from hypothetical.

Rusted-on supporters could turn

What's different about this particular manifestation of white supremacist politics is that it might just turn otherwise-rusted-on supporters against Mr Trump when it comes to the future of one of the United States' most important international relationships. It's doubtful the President foresaw such a possibility.

Mr Johnson, a leader also known for his racist politics, likely also didn't anticipate such a turn of events.

It's crucial we recognise the pervasiveness of white supremacy in today's politics, and that we grapple with its consequences — even when they might surprise us.

When it comes to Brexit and Ireland, for once, white supremacy might not work in favour of racist leaders. But that doesn't make it any less horrifying.

Dr Emma Shortis is a research fellow at RMIT University's EU Centre of Excellence, Social and Global Studies Centre.