When the time comes to write the history of Brexit, the historian will face a familiar question. What drives historical change? Is it deep, structural forces, boiling away beneath the surface of society? Or accident and contingency: small moments which, like the switches on a railway line, need only a touch to set the train of events speeding to a different destination?

It’s a recurring dilemma. The English Reformation, the First World War, the fall of the Berlin Wall: all had powerful, long-term causes. But what if Henry VIII had been blessed with less active loins? What if Franz Ferdinand’s car had taken a different route? What if an East German official had not misunderstood his orders and opened the crossing by mistake? Or, to return to Brexit: what if the largest pro-European party in Britain had not accidentally elected a Eurosceptic as leader, less than a year before a referendum on membership?

“Accidentally”? In one respect, yes. The election of Jeremy Corbyn had its own structural dynamics, but they had little to do with Europe. For those who cheered his victory in 2015, Corbyn’s views on Europe were an irrelevance; but their effects have been among the most profound of his leadership.

Like his mentor, Tony Benn, Corbyn had been a Eurosceptic since the 1970s. He voted against membership in 1975, opposed Labour’s pro-European turn in the 1980s and condemned the creation of the European Union in 1993 as “a bankers’ Europe” that would “endanger the cause of socialism”. He viewed the EU as an undemocratic, capitalist project that aspired to a “European empire”; a “military Frankenstein” that was “subservient to NATO , ” governed by “an unelected set of bankers”. Together with John McDonnell, Denis Skinner and Richard Burgon’s Uncle Colin, Corbyn was one of only 10 Labour MPs to vote against the Lisbon Treaty on its third reading in 2008. Even during the leadership contest in 2015, he refused to rule out voting to Leave. Since 2015, Corbyn and McDonnell have staffed their offices with Eurosceptics like Seamus Milne, a critic of the EU’s “brutal authoritarianism”, and James Meadway, who once wrote that it was impossible “in good conscience” to support “an institution so manifestly and comprehensively opposed to democracy”.

Yet Corbyn’s Euroscepticism differed in one important respect from that of Benn: its intensity. Whereas Benn demanded a “national liberation struggle” to free Britain from colonial status, Corbyn’s passions lay elsewhere: in the struggles against US imperialism in Latin America; against apartheid in South Africa; and against Israel in the Middle East. He rarely spoke in the big parliamentary debates on Europe and did not even vote on Cameron’s Referendum Bill. As Jason Farrell and Paul Goldsmith conclude in their book How to Lose a Referendum, Corbyn “simply wasn’t that interested in the Europe question.” That made it possible for him to back Remain in 2016, under pressure from his shadow cabinet, but it put him in the extraordinary position of “leading a campaign he didn’t much care about.”

For the official Leave campaign, Corbyn’s rise was an unexpected boon. Its chief executive, Matthew Elliott, called his election “a massively good day for Vote Leave , ” which disabled “the Labour In campaign [and] enabled the Labour Leavers to project a stronger image than their numbers in parliament actually deserved”. The campaigning group “Left Leave” plastered its website with Corbyn quotes, while Leave.EU tweeted clips of him castigating the EU. Throughout the campaign, a leader who would blaze across the electoral trail in 2017 seemed bored and disengaged, cheerfully admitting on Channel 4 that “I’m not a huge fan of the European Union.” Three weeks before the vote, polling found that half of Labour voters were still uncertain whether the party was for or against membership.

If Corbyn was unenthusiastic, his office seemed openly hostile. Labour Remainers complained of articles and speeches blocked, media appearances cancelled and shadow ministers kept off the airwaves. The result was not only to knock out Britain’s second biggest party as a campaigning force, but to cast the referendum as an internal struggle within the Conservative Party. In the absence of a social democratic case for membership, a vote for Remain became a vote for David Cameron. Though most Labour voters still backed Remain, internal polling suggested that support dropped by as much as ten points over the campaign.

This does not mean that Brexit happened “because of Jeremy Corbyn.” Brexit was a fire lit by the Conservative Party, out of kindling that had piled up over decades. Yet, if nothing else, the Corbyn effect shut down one of the hydrants on which the Remain campaign had hoped to draw, reducing what might have been a flood to a trickle. The waters have remained at a low ebb ever since.

In the period since 2016, Corbyn has whipped his MPs to trigger Article 50; denounced EU rules on state aid; sacked frontbenchers who backed the single market or a second referendum; confirmed that a Labour government would press on with Brexit after an election; and run down the clock on a “People’s Vote.” In so far as Labour has an alternative policy offer, it is to promise a better Brexit: a “Jobs-First Brexit , ” which repudiates only the manner of Britain’s departure. There are good, strategic arguments for that position, not least when so many Labour seats backed Leave. Yet it is hard to imagine any alternative leader for whom Brexit was of so little concern—or who would be so readily forgiven by the party’s Remain-supporting membership. The effect has been to anaesthetise an important bloc of Remain opinion, while ensuring that pro-European voices in parliament speak only from the backbenches.

So was this simply an accident of history? Not quite. Corbyn’s Euroscepticism was hardly a secret in 2015; yet a pro-European membership elected him anyway, in full knowledge that a referendum was approaching. A year later, amidst the ashes of that vote, they re-elected him by a landslide. If Corbyn’s Euroscepticism was not the cause of his popularity, nor was it a barrier to his success—even when his opponent, Owen Smith, made it the centrepiece of his campaign.

As this reminds us, enthusiasm for Europe has often been skin-deep: a “twibbon” to be displayed on social media, not a commitment that might influence political choice. Eurosceptics do not wear their convictions so lightly. Though not hostile to membership, Labour’s pro-Europeans were unwilling to make sacrifices to defend it; and in that respect, at least, the triumph of Labour’s Eurosceptics embodies something far deeper and more powerful than chance.