The resignation last week of Britain’s ambassador to the United States was a reminder of the sorry state of the special relationship—as if one were needed. The strains on the allegiance that, with a healthy dollop of self-flattery, we Brits think is essential for the United Kingdom, the United States, and even the rest of the world, are numerous.

There’s Donald Trump himself, whose isolationist instincts and contempt for protocol make his presidency heavy on headaches and light on opportunities for British officials. There’s Brexit, which was supposed to be an opportunity to strengthen Britain’s ties beyond Europe but has become an undignified distraction that leaves the United Kingdom diminished in the eyes of just about everyone. And there’s the ongoing row over Huawei and whether the British government should, against the wishes of the U.S. government, involve the Chinese in domestic 5G infrastructure.

But there is a far bigger threat to the special relationship that receives far less attention. It comes in the unlikely form of a scruffy septuagenarian socialist who also happens to be the Leader of Her Majesty’s Opposition.

Jeremy Corbyn’s ascent from far-Left backbencher to leader of the Labour Party in 2015 was one of the most astonishing surprises in recent British political history (and there have been quite a few). Two years later, he exceeded expectations once again with a surprisingly strong showing in the general election. Today, he is bogged down in two internal rows that might yet topple him: one over his stance on Brexit, the other over Labour’s stomach-churning anti-Semitism problem.

And yet he could still end up in Downing Street. British politics has become an unpredictable, unprecedented four-horse race, and the Conservatives’ own internal strife and vanishingly small Parliamentary majority means an election might well be around the corner, which could easily end with a Labour government.

Why, you might be wondering, would that be such a disaster from an American perspective? After all, this Conservative administration is hardly a model of good governance, and the special relationship has survived periods when the two governments don’t see eye to eye.

But what it may not be able to survive is a Prime Minister whose fundamental orientation is so anti-American that vital U.S.-UK intelligence sharing could be suspended under his rule. Intelligence cooperation between London and Washington has formed the bedrock of the Five Eyes alliance since the 1940s, enabling unprecedentedly close ties among the main Anglophone countries and underpinning the U.S.-UK special relationship even at its most fractious. Corbyn’s election could change all that, jeopardizing cooperation on counterterrorism, counterespionage, and a host of other security priorities.

For Jeremy Corbyn is not just another Labour leader. Corbyn’s long history of controversial statements, dubious associations, and anti-Western apologetics has been well documented since his rise to leadership in 2015. Yet it’s still worth digging deeper, beyond the soundbites that have emerged piecemeal over the past four years, to take a closer look at the ideas Corbyn has been marinating in for decades.

These are ideas that no previous Prime Minister has taken remotely seriously, and they go far beyond Corbyn’s personal animus toward Donald Trump. They are a set of views that could cause the whole U.S. government to question whether the United Kingdom is a country it can safely continue doing business with.

Corbyn’s electoral appeal might be domestic, but his preoccupation has always been foreign policy. And if Corbynism has a single guiding principle, it is that the actions of the United States and its allies are everywhere and always to be opposed.

It was hardly surprising that Jeremy Corbyn turned down his invitation to the state dinner with Donald Trump on his recent visit to London. The Labour leader isn’t alone in thinking the U.S. President beyond the pale, and making that clear won’t have cost him many votes.

But you get a clue as to his worldview from who he is willing to associate with—and what he chooses to say and do. He was happy to take money from the Iranian regime to host a show on their propaganda channel Press TV. He invited convicted IRA terrorists to Parliament shortly after the organization had murdered five people with a bomb at the Conservative Party Conference in Brighton in 1984. He called Hamas and Hezbollah his friends, describing the latter as “an organisation that is dedicated towards the good of the Palestinian people and bringing about long-term peace and social justice.” He blamed the Russian invasion of Ukraine on NATO provocation. He called Hugo Chavez an “inspiration to us all.” He put his name to a motion that congratulated left-wing journalist John Pilger “on his exposé of the fraudulent justifications for intervening in a ‘genocide’ that never really existed in Kosovo.”

From 2011 to 2015 Corbyn chaired an organization called Stop the War. That is a misnomer, for it only ever opposes military action undertaken by the West. Sometimes, in fact, it supports violence, as in 2005, when it backed “the legitimacy of the struggle of Iraqis, by whatever means they find necessary” to secure the withdrawal of coalition troops from the country. And though Stop the War helped to organize the mainstream protests against the Iraq War in London in 2003, the coalition has long been run by fringe left-wing parties, the Socialist Workers Party and Communist Party of Britain (CPB) among them.

Corbyn was preceded as Stop the War chair by an aristocratic Scottish Stalinist called Andrew Murray. As recently as 2016, he was a member of the CPB. In 2003, he expressed his support for North Korea when he wrote: “Our party has already made its basic position of solidarity with People’s Korea clear.” Today he is employed as an adviser to Jeremy Corbyn within the Labour Party.

Astonishingly there is room for more than one Stalinist in Corbyn’s top team. Seumas Milne, a co-conspirator of Andrew Murray and former Guardian columnist, is the Labour Party’s Head of Communications and Strategy—and arguably Corbyn’s most powerful adviser. As a young man Milne wrote for Straight Left, a publication that backed the Soviet invasion of Afghanistan.

No wonder the CPB didn’t put forward any candidates in the 2017 general election, instead advising its supporters to back Labour. Jeremy Corbyn, it seems, was close enough to the real deal. After all, for years Corbyn wrote a column for the Morning Star, the newspaper published by the CPB which the Labour leader has called “the most precious and only voice we have in the daily media.”

Like Corbyn, Milne has a suspicion of NATO that makes Trump look like a dedicated Atlanticist. In 2014, he wrote, “Western aggression and lawless killing is on another scale entirely from anything Russia appears to have contemplated, let alone carried out—removing any credible basis for the US and its allies to rail against Russian transgressions.” For Milne, “Russia’s subsequent challenge to western expansion and intervention in Georgia, Syria and Ukraine” helped to provide a “check to unbridled US power.” In 2007, he used his Guardian column to stick up for communism as follows: “For all its brutalities and failures, communism in the Soviet Union, Eastern Europe and elsewhere delivered rapid industrialization, mass education, job security and huge advances in social and gender equality.”

Milne’s responses on the pages of the Guardian to the terrorist attacks of the early 2000s are as sickening as they are predictable. America was “reaping a dragons’ teeth harvest they themselves sowed,” he wrote after 9/11. The London bombings were “driven by worldwide anger at US-led domination and occupation of Muslim countries.” For Milne, “The only surprise was that the attacks were so long coming.” Echoing Stop the War, he described the anti-coalition insurgency in Iraq as the country’s “real war of liberation.”

If you think it unfair to pin Corbyn for the words of his advisers, consider the clumsily expressed views of the man himself. In a foreword to a 2011 edition of Imperialism by J.A. Hobson, the Labour leader—then an obscure backbencher—explained his view of modern history:

Compare this with Corbyn’s characterization of the other side of the Cold War:

The contrast is typical of the way Corbyn and his coterie see things. The assumption that barely needs stating is the wickedness of America, capitalism, and imperialism. The other side of any conflict is to be sided with. And the crimes of anyone—from Stalin to Hamas—brave enough to stand up to American tyranny are to be excused, explained or simply ignored.

Elsewhere in Corbyn’s foreword we are treated to the claim that the War on Terror had “as much to do with economic interests as any notion of ‘security’” (those scare quotes reveal as much as the words themselves), and the idea that the political appeal of capitalism is fading thanks to “a combination of Islamic opposition and the radical popular movements of landless and poor peoples in many poor countries.”

It is tempting to dismiss all of this as naive nonsense. Indeed, that is what much of it is. But sadly the provenance—the Leader of the Opposition and his senior advisers—means it must be taken seriously, including by Britain’s closest ally. And when they do so, U.S. officials might reconsider even the most essential forms of cooperation—counter-terrorism, national security and intelligence sharing.

If the special relationship means anything at all, it isn’t bromides delivered at joint press conferences. It’s close collaboration on these especially sensitive areas. But such collaboration cannot be taken for granted. American leaks caused the UK to briefly stop sharing intelligence after the Manchester bombing in 2017. A Corbyn government would likely mean a more serious deterioration.

Ironically, such a move would confirm the Corbynistas’ worst conspiratorial suspicions about America’s security services, while Donald Trump’s unpopularity in the United Kingdom means British voters aren’t quite as worried about upsetting the American government as they might have been a few years ago.

But who could blame Washington for downgrading its relationship with Britain were Corbyn to win an election? He and his acolytes weren’t simply principled opponents of the Iraq War; they were cheering on the resistance. They weren’t opposed to a particularly U.S. strategy during the Cold War; they wanted the other side to win. To them, America is the problem, not the solution.

And so a potentially fatal blow to the special relationship would be one of the more inevitable tragedies of a Corbyn government. The more immediate problem is that the shabby track record that would likely cause the U.S. government to run a mile from Prime Minister Corbyn does not appear to have the same effect on British voters.