On the evening of 16 May 2007, John McDonnell conceded defeat. The left-wing Labour leadership candidate had won only 29 MP nominations, 16 short of the number he needed to make the ballot. “I know how angry many of you are, but I would ask you to stay in the party and fight,” McDonnell wrote in a letter to his supporters as Gordon Brown became the new Labour leader – and thus the prime minister – unopposed. “Don’t mourn, organise,” McDonnell concluded.

“It’s pathetic,” the BBC presenter Andrew Neil said to the Labour MP Diane Abbott on This Week. “Your lot can’t even muster 45 backers.”

In 1996, Tony Blair declared: “You really don’t have to worry about Jeremy Corbyn suddenly taking over.” Yet, two decades later, Corbyn is the leader of the opposition, McDonnell is the shadow chancellor and Abbott is the shadow home secretary. After Labour eradicated the Conservatives’ majority at the 2017 general election, Blair was forced to admit that Corbyn could indeed become prime minister.

Jeremy Corbyn’s rise to the leadership is often portrayed as the political equivalent of an X Factor victory: a shock, a sensation. However, its roots lie in the years that followed McDonnell’s defeat in 2007. Rather than representing the beginning of a new movement, Corbyn’s victory in the 2015 Labour leadership contest was, in many respects, the culmination of one.

The unimpeded transfer of power from Blair to Brown appeared to confirm New Labour’s hegemony. Brown cast himself as a more social democratic leader but, in reality, he continued to accept the fundamentals of the Thatcherite settlement. “When Gordon was leader, I can’t think of a single moment when he took that group [the hard left] seriously,” Stewart Wood, a Labour peer and former Brown aide, told me.

He should have. By 2007, the left had led the anti-war movement and been strengthened by the “awkward squad” of trade union general secretaries. These included Derek Simpson and Tony Woodley of Unite, Billy Hayes of the Communication Workers Union, Bob Crow of the National Union of Rail, Maritime and Transport Workers and Mark Serwotka of the Public and Commercial Services Union.

“That generation had been more pro-Tony Benn than Neil Kinnock,” Simon Fletcher, the former chief of staff to both Ken Livingstone and Jeremy Corbyn, said when we met recently near Westminster. “They had been central to industrial struggles.”

In parliament, the left, or “hard left”, assembled around the Socialist Campaign Group (founded in December 1982 by supporters of Benn), which McDonnell chaired. Its 13 MPs were defined by their opposition to Western foreign policy (Corbyn and McDonnell voted against every major Anglo-American intervention), their support for widespread nationalisation, high taxes and substantial redistribution and their Euroscepticism (the EU was considered a capitalist or neoliberal club). Labour’s “soft left”, by contrast, was more moderate in its criticism of New Labour (some of its members served in government) and embraced the European project as a bulwark against Thatcherism.

Once it became clear that Gordon Brown would not face a challenger from the right of the party (John Reid had been tipped as a possible contender), his campaign team worked relentlessly to keep McDonnell off the ballot. Against the left-winger, some feared, Brown would appear the candidate of continuity rather than of change.

Today, McDonnell’s friends believe that he would have exceeded expectations had he made the contest. I asked the super-activist Jon Lansman, the founder and chair of Momentum, who managed Tony Benn’s unsuccessful 1981 deputy leadership campaign, for his memories of the period. “My wife had died and I had dropped out of politics for a few years; 2007 was when I came back into activity in the Labour Party,” he said. “Had John [McDonnell] been on that ballot paper, he would have won a very significant level of support within the trade unions.” But Lansman conceded: “In the constituency parties, it would have been much tougher. There had been no opening up.”

The 2007 deputy leadership contest, however, hinted at the left’s potential strength. Jon Cruddas, then the little-known MP for Dagenham, fought a campaign defined by its critique of the failings of New Labour’s political economy. He eventually finished third out of six candidates, behind Harriet Harman and Alan Johnson, but notably led in the first round. The desire for alternatives to Blairism had been demonstrated.

In late 2006, Corbyn floated the possibility of standing for the deputy leadership. “There needs to be an anti-war candidate,” he said. “None of the existing ones are. Even [Peter] Hain and [Jon] Cruddas voted for the war [in Iraq].” In the end, he nominated Hilary Benn, the son of his mentor Tony Benn, but Corbyn had shown ambition.

At the time Brown became prime minister, few believed that his boast to have ended “boom and bust” would be challenged. There was wide agreement on the right and left that radical economic transformation was no longer possible – or perhaps even desirable – in an era of globalisation. After the longest sustained period of growth for 200 years, the free market appeared to have beaten back all threats to its rule.

In May 2007, Oliver Letwin, the chair of the Conservative policy review, declared: “Instead of being about economics, politics in a post-Marxist age is about the whole way we live our lives; it is about society… As David Cameron put it… ‘It’s not economic breakdown that Britain now faces, but social breakdown.’”

But economic breakdown was what the UK was heading for. In September 2007, Britain endured its first bank run for 141 years as Northern Rock tottered, unable to raise sufficient income to repay its loans. The crisis led to the bank’s nationalisation in February 2008. Brown defended this as a pragmatic intervention, but the taboo over state ownership had been lifted. The effects of the financial crisis, the most severe since the Great Depression of the 1930s, led to something that would have been unthinkable in the Blair years: the nationalisation of the commanding heights of the banking sector (including 81 per cent of the Royal Bank of Scotland and 43 per cent of Lloyds).

The political ramifications were no less significant. As John McDonnell told me by email: “The crash was pivotal in shattering confidence for a period in the operation of capitalism and demonstrating the inherent crisis-ridden nature of the system… As people sought an explanation for the crisis, they became open to the traditional ideas of the left, ranging from Marx to [John Maynard] Keynes and [John Kenneth] Galbraith, to the modern analyses of economists like Thomas Piketty.”

In a new age of economic scarcity, with GDP falling by 6 per cent from 2008 to the end of 2009, dormant policy questions were resurrected. New Labour had used the proceeds of growth to increase public spending without significantly raising taxes on the highly paid and rich. Yet the disappearance of growth negated this model. The financial crisis intensified the hardship that many people were already experiencing. From 2003 onwards, 11 million low-to-middle earners experienced no rise in their real incomes. In a country that remained almost as unequal as it had been in the 1980s, the gains from growth all too seldom flowed downwards.

The Labour shadow cabinet minister Jon Trickett, one of Corbyn’s closest allies, told me: “You cannot have a centre-left party whose demographic base is meant to be lower- and middle-income earners administering the Thatcherite settlement because, in the end, you come into conflict with your own base. And that had already begun to happen by 2005.” (Labour was re-elected for a third term under Blair with just 35.2 per cent of the vote, the lowest share of any majority government in UK history.)

After the Iraq War further eroded New Labour’s moral legitimacy, and the crash its economic credibility, its last remaining pillar – political supremacy – collapsed as well. Many party members had tolerated Blairism because it was electorally successful. But when Labour’s poll ratings sank, the project lost its unique attribute.

“Tony Blair never had troops on the ground committed to his project,” Jon Lansman told me. “Labour Party members never believed in privatising the NHS, they never believed in academy schools, they never believed in the Iraq War. They were never with Blair on policy. That’s why democracy was prevented. As soon as the members had the power to elect the leader, it was going to be a battle for the votes of the left [of the party] and the centre, not for the right-wing vote.”

As the longest period of Labour government in history ended with the party winning just 29 per cent of the vote at the 2010 general election, its 13 years in office were increasingly regarded by many members and activists with shame, rather than pride. Crucially, the Labour left accepted no blame for the misdeeds of this era. “The left had clean hands,” Trickett told me. “None of their fingerprints can be found at the scene of any of the crimes that were committed from 1979 or from 1997 onwards. They were arguing for an alternative.”

***

In the 2010 leadership election, which was the party’s first full contest in 16 years, the left was determined to field a candidate. As the chair of the Socialist Campaign Group of MPs, John McDonnell put himself forward again. But he faced competition from an unexpected source: his fellow left-winger Diane Abbott. The Hackney North MP stood because she believed that it was unacceptable to have a contest in which “there are no women”.

On 9 June 2010, McDonnell withdrew, having attracted only 16 nominations. The rival bids reflected and reinforced the uneasy relationship between Abbott and McDonnell, which endures to this day. “John gets very annoyed with Diane’s activities from time to time, and she has a very negative view of him as well,” a senior Labour source said of Corbyn’s closest allies. “Diane will say privately to Jeremy that John is more interested in himself than he is in Jeremy.”

Abbott, who had received 11 nominations at the time of McDonnell’s withdrawal, became the first left-winger to feature in a Labour leadership election since Tony Benn in 1988. She made it on to the ballot paper after David Miliband and his allies lent her their support. This prevented an all-male, all-white contest. Miliband hoped that Abbott’s inclusion would weaken the prospects of his left-leaning brother, Ed, who was emerging as an ardent critic of the Blair years. The precedent of MPs nominating a candidate for whom they had no intention of voting was established.

In the minds of Labour’s senior politicians, the subsequent contest confirmed the weakness of the left. Abbott finished fifth out of five candidates after being eliminated with 7.4 per cent of the vote. But for those who looked closely enough, the left’s true strength was evident. Had the contest been conducted under the one-member-one-vote system, with Labour MPs deprived of their 33 per cent share, Abbott would have finished third, ahead of Ed Balls and Andy Burnham. And, as Lansman noted, she “had baggage, she had enemies on the left”. (Some never forgave her for sending her son to the private, fee-charging City of London School.)

The left was also heartened by Ed Miliband’s victory over David, the anointed heir of New Labour. (The younger Miliband won by 1.4 per cent.) “Our intervention was pretty decisive in that election,” Lansman recalled. “In previous years, the left might not have supported Ed, who came completely out of the Brownite stable. But they did. People transferred from Abbott to Ed Miliband. Ed’s election opened up a space for the left to be able to argue its case.”

Unlike Blair and Brown, Ed Miliband did not define himself against the left and aligned with it on the Iraq War, privatisation and taxation. He spoke warmly of Tony Benn and other left-wingers, some of them friends of his late father, the Marxist academic Ralph Miliband. At times, the Blairites appeared determined to drive the left out of the party, transforming Labour into a version of the US Democrats. Under Miliband, the left was part of the family once more. Abbott became shadow public health minister, Simon Fletcher became Miliband’s trade union liaison officer and the Labour leader sought the advice of the Guardian columnist Seumas Milne, who now serves as Corbyn’s strategy and communications director.

“Ed Miliband lightened the mood within the Labour Party,” McDonnell said. “He allowed a more free-ranging debate on the analysis of our society, the issues that needed to be confronted and the policies. This allowed the long march back from the neoliberal capture of some of Labour’s thinking.”

It was not only developments within Labour that favoured the left. The decision by the Liberal Democrats to form a coalition government with the Conservatives in 2010 left many of their supporters, particularly those who were against the Iraq War, feeling betrayed. James Schneider, who later became Corbyn’s head of strategic communications, left the party that year (having been president of the Oxford University branch). The Lib Dems’ decision to triple university tuition fees from £3,000 to £9,000, in violation of their election pledge, intensified the damage.

The coalition government’s pursuit of austerity led to an upsurge of radical activism and industrial militancy. In November 2010, Len McCluskey, a former Militant supporter, was elected general secretary of Unite, reflecting the leftward trajectory of Labour’s largest union affiliate. New anti-capitalist movements, such as UK Uncut and Occupy London, were established. At major demonstrations, Corbyn, McDonnell and Abbott were reliably among the speakers.

Emma Rees, who attended UK Uncut’s actions as a primary school teacher, and who is now Momentum’s national organiser, told me, “We were presenting a common sense idea: that corporations should pay their taxes. It was the precursor to Labour’s economic approach [under Corbyn]. This isn’t about being radical left. This is about common sense politics that most people agree with.”

As a senior Labour source noted of the new left-wing movements: “They were quite post-ideological. You’ve not got the Cold War, you’ve not got the Soviet Union, people are not hidebound by all their views on Stalin, Trotsky, Mao, Gramsci. They are people who want to change the world and are socialists in most cases, but they’re not defined by ideological divisions. They just get on with things.”

***

During his leadership, Ed Miliband sought to harness such radicalism. He addressed a 2011 anti-austerity demonstration (to the consternation of some of his MPs). Asked in Brighton by a Labour member, “When will you bring back socialism?” he replied, “That’s what we are doing.” Interventionist policies, such as a freeze in energy prices, a cap on rent increases and a “mansion tax”, were adopted. When I recently met Miliband in his spacious parliamentary office, he told me: “I was always struck when we would propose policies such as the energy price freeze, rent caps [and] ‘use-it-or-lose-it’ land controls that all of the polling would show a) that people supported the policy, but b) that, if anything, people wanted us to go further. That’s why the ‘back to the 1970s’ Daily Mail attack on me, or on Jeremy for that matter, never really worked, because of where the public were.”

Yet Miliband ultimately disappointed the left. Rather than vowing to end austerity at the 2015 election, as activists hoped, Labour promised to moderate it. “We lost the battle around austerity,” Jon Cruddas, who led the party’s policy review under Miliband, told me. “I remember when we launched the 2015 manifesto, even that day, the centre rewrote it after the Clause V meeting [in which the manifesto’s contents were formally approved] to give it a more fiscally orthodox tone.”

The “austerity page”, as it became known, committed Labour to no “additional borrowing” for spending commitments and to cutting the deficit “every year”. I was told Miliband now regrets not sacking Ed Balls, who championed fiscal restraint, as shadow chancellor (a claim Miliband denies).

When I interviewed John McDonnell during the 2015 election campaign, he warned that left-wing MPs would force Miliband to abandon austerity if he persisted. The Times splashed on the story but when one of the paper’s journalists questioned a Labour aide on the subject, he replied, “John McDonnell?” – his tone implying that the left was irrelevant.

In Scotland, meanwhile, where Miliband had hoped his interventionist policies would resonate, it was the SNP that surged (winning 56 seats while Labour was left with one). For some in Labour, this was further evidence of the desire for radical alternatives to the status quo.

But after the election, the dominant media narrative was that Miliband had lost to the Tories because he was too left-wing. All of the initial leadership candidates, to varying degrees, accepted this argument. Even Andy Burnham, who had previously pitched himself as a left-wing alternative to Ed Miliband, tacked to the right, condemning Labour’s proposed “mansion tax” as the “politics of envy”, refusing to rule out further welfare cuts and rejecting trade union donations. Burnham pursued this strategy in the belief that “the left had nowhere else to go”. But soon they would.

A political vacuum had emerged, but the left was seemingly incapable of filling it. McDonnell had twice tried and failed to get on the leadership ballot paper. After suffering a heart attack in 2013, he had resolved to avoid another strenuous bid. Diane Abbott had finished last in the 2010 leadership contest and was committed to her London mayoral campaign. Both Jon Trickett and Ian Lavery, the former National Union of Mineworkers president (who is now the Labour Party chair), declined to stand. Those who remained were considered either too old or too young (eight of the left’s cohort, reflecting the shift under Miliband, had been elected in 2015).

“What about if I stand?” With those words on 27 May 2015, Jeremy Corbyn offered himself as the answer to the left’s conundrum. His suggestion was met with silence by colleagues. Few had considered the Islington North MP to be a potential candidate. Among local members in London, talk had turned to the likelihood of the 66-year-old standing down at the next general election. But a week later, after a bullish response from left-wing activists, the Socialist Campaign Group resolved that Corbyn would stand as its candidate.

If Corbyn was to achieve the requisite 35 MP nominations, he needed help. The left could provide no more than 20. Like Abbott in 2010, Corbyn required the kindness of strangers. At 11:58am on 15 June, with two minutes to spare, Corbyn made the ballot (an emotional McDonnell having fallen to his knees and begged for nominations). The 36 MPs who nominated Corbyn had different motives for doing so. Some were sincere supporters; others hoped to “broaden the debate”; still more (such as Frank Field) aimed to discredit the left through defeat.

Corbyn’s amiable personality was also a factor. “Jeremy was unique. He did not have enemies on the left,” Jon Lansman said. “He didn’t even have enemies in the centre. He was a decent, genuine, principled person, and that’s why he got on the ballot and no one else would have.”

Simon Fletcher, who was Corbyn’s chief of staff in the leadership contest, said: “Jeremy was rarely the star speaker at protests. He was just there at everything. It created a personality of sincerity, which is not at all artificial. That is genuinely what he’s like.”

What all of those who nominated Jeremy Corbyn – including Corbyn himself – had in common was that they did not believe that he would win. But some people knew better. Max Shanly, a 25-year-old left-wing Labour activist, predicted Corbyn’s victory an hour after his candidacy was announced. “We’re going to run a campaign that will inspire thousands of people to get involved in politics for the first time and change the future of this country,” he told Simon Darvill, the then chair of Young Labour. “Everyone laughed,” Shanly recalled.

The New Statesman’s Stephen Bush also predicted that he would do significantly better than expected and tweeted to this effect. For the first time in Labour’s 115-year history, the new leader would definitively be chosen by party members.

***

The “butterfly effect” is the term coined by the US meteorologist Edward Lorenz to describe how small events can have larger consequences (so named for hurricanes triggered by the flapping of a butterfly’s wings). On 22 February 2012, one such moment occurred. In the Palace of Westminster’s Strangers’ Bar, the Labour MP for Falkirk, Eric Joyce, headbutted and punched the Conservative MP Stuart Andrew and struck several others, complaining that the establishment was “full of fucking Tories”. He had been drinking.

The following day, having been arrested, Joyce was suspended from the party. Joyce announced that he would not seek re-election, and Unite was accused of manipulating the subsequent Labour candidate selection. Len McCluskey, the Unite general secretary, championed the candidacy of his close friend Karie Murphy, who is now the director of Corbyn’s office.

Miliband responded to the controversy by seeking to weaken the trade unions’ influence, most notably in Labour leadership contests. The electoral college – under which affiliated union members, MPs/MEPs and party members each held a third of the vote – was replaced by a one-member-one-vote system. Miliband’s motives were democratic, rather than ideological. But some on the left immediately saw the potential to benefit.

Simon Fletcher, who was closely involved with the rule change as Miliband’s union link man, said: “The prize was one-member-one-vote, the removal of MPs’ ‘golden share’. It was an absolute leap forward in democratic terms, and it completely evened out the playing field.” No longer would a lack of support among MPs (which hindered Abbott in 2010) be an obstacle to victory for the left.

The new system was accompanied by an increase in the nomination threshold from 12.5 per cent to 15 per cent, designed to obstruct maverick challengers. Ed Miliband’s team had proposed a higher threshold of 20 per cent but it was overruled by the party’s right, who feared that Blairite candidates could suffer.

The final change was the introduction of a “registered supporters” system in which individuals paid £3 to vote in any leadership contest, potentially opening the party up to entryism or outside manipulation. Blairites had long argued for such a move in the hope that it would attract “centrist” members of the public who would counterbalance left-leaning members. But the new rules energised Corbyn’s campaign. It was an instance of what Hegel called “the cunning of history”: individuals’ actions serving epochal forces of which they are only dimly aware.

“The left in parliament had dwindled, but there were mass movements outside,” Fletcher recalled. “There were a lot of politically motivated people who, given the opportunity to vote for a candidate they believed in, would take it.”

For activists such as Emma Rees, Corbyn’s candidacy provided a reason to join Labour for the first time. “Jeremy and John [McDonnell] very successfully managed to keep one foot in campaigning movements outside parliament and the other foot inside,” she said. “Because they had those links outside Labour, once Jeremy got on the ballot there was always going to be an explosion of energy and innovation.”

In some cases – as with Greece’s Syriza, Spain’s Podemos and France’s Jean-Luc Mélenchon – the leftist explosion took place outside the political mainstream. In Britain, it happened within the venerable Labour Party. Young recruits were joined by older returners who had left the party during the Blair years. The membership swelled from 200,000 in May 2015 to 299,755, with a further 189,703 affiliated supporters and 121,295 registered supporters (at more than 600,000, Labour’s membership is now among the highest in western Europe).

Emily Thornberry, who nominated Corbyn, and who became shadow foreign secretary in 2016, distilled the membership’s mood when we met recently in Portcullis House, Westminster. “Labour members were told: ‘We have to continue to compromise to get power. We can’t be true to ourselves. We have to triangulate.’ If that works and we get power, then fine, but if we don’t get power, there comes a time when the membership says, ‘Do you know what? This isn’t working. Why don’t we go back to who we are and regroup, because we’ve lost our way with all this triangulation?’”

This sentiment propelled Corbyn to a landslide victory on 12 September 2015 (he won 59.5 per cent of the vote in the first round). As his fellow MPs plotted to bring him down, activists embraced Corbyn as the only unambiguously anti-austerity, anti- war and pro-immigration candidate.

Two years later, Corbyn remains unassailable. In 2016, he was re-elected with 61.8 per cent of the vote against Owen Smith after a failed MPs’ coup. At the 2017 general election, Corbyn helped increase Labour’s vote share from 30.4 per cent to 40.0 per cent, the biggest rise since 1945. The party gained 30 seats (262) and deprived the Tories of a majority.

***

None of this was inevitable when Corbyn was first elected, with the overwhelming support of members but the backing of just 15 MPs. He could have departed in 2016, when the Parliamentary Labour Party passed a vote of no confidence by 172 votes to 40, and 65 shadow ministers resigned in protest at his leadership. But because of his sense of obligation to his supporters and the absence of a viable left-wing successor, he persisted. As Fletcher said: “Jeremy’s not going to go if he thinks he’s just handing the party to the right.”

During an appearance on The Andrew Marr Show on 10 July 2016, Corbyn was asked about the “enormous personal pressure” he faced and whether he had “a wobble”. He replied: “There’s no wobbles, there’s no stress, there’s no depression. I’ll tell you what real stress is. Real stress is when you can’t feed your kids. Real stress is if you don’t know you’ve got a job next day. Real stress is if your landlord is going to evict you from your home. That’s what real stress in our society is.”

Corbyn’s answer exemplified the spirit of altruism and self-sacrifice that inspires such loyalty from party members (the most ardent draw comparisons with another JC). The Labour leader was aided by long-standing comrades, such as McDonnell and Abbott, and by the backing of soft left MPs such as Thornberry and shadow education secretary Angela Rayner (loyalty Corbyn has repaid)

I asked McDonnell what sustained the left during the MPs’ revolt of 2016. “The overwhelming enthusiasm and determined support we received from a loyal activist base wherever we appeared,” he said. “Plus, a sheer Scouse bloody-mindedness that this was a historic opportunity that nobody would take away from us without a fight.”

When Theresa May called an early general election in April, Labour’s dismal poll ratings made even ardent supporters fear that the party would suffer its worst defeat since 1983 (when it won 209 seats) or 1935 (when it won 154). Its performance in the local elections in May seemingly confirmed that it was heading for a historic defeat.

Yet, just as in the 2015 leadership contest, Corbyn exceeded expectations. His insurgent election campaign upended long-held assumptions about British general elections: that young people don’t vote, that campaigns don’t make a difference and that opposition leaders never recover from a bad start or dire personal ratings.

“Labour did something next to impossible: Jeremy Corbyn got a second chance to make a first impression,” YouGov’s Marcus Roberts, a former party strategist, told me. Corbyn’s aides presciently argued that his standing would improve once the election broadcasting rules took effect. They prioritised footage of the leader addressing rallies over interviews with the largely hostile newspapers.

McDonnell said: “Because I was on the back benches for 20 years, the Tories and some in our own party underestimated not just our determination to win but also the creative professionalism of our team.”

It was the party’s manifesto – For the Many, Not the Few (a reference to Shelley’s poem “The Masque of Anarchy” – that Corbyn regarded as the true “star of the campaign”. Written by the former trade union official Andrew Fisher (whom Labour MPs once sought to have expelled from the party), it proposed popular, populist policies such as the abolition of student tuition fees, universal free school meals and the renationalisation of the railways, the water industry, the energy grid and Royal Mail, as well as higher taxes for the top 5 per cent of earners and increased taxes on corporations. If this was socialism, it transpired, it was what a large number of voters wanted.

***

Labour’s 2017 election performance has put the party within reach of government once more. Jeremy Corbyn needs a swing of around 5 per cent to achieve a Commons majority (326 seats) at the next general election – which aides unambiguously state he will fight, even if the present parliament endures until 2022. Should Labour gain 29 Conservative seats, it will surpass the Tories as the single largest party. After a precipitous decline in home ownership – historically the transmission belt to Toryism – the Conservatives are struggling to sell capitalism to a generation without capital.

However, Corbyn’s challenge remains daunting. He must navigate the epic issue of Brexit, which divides both Labour voters and MPs. Corbyn and McDonnell are Eurosceptics who have often spoken of the EU’s single market as an obstacle to socialism. Other MPs, however, regard the European project as an essential precondition of economic prosperity and social justice. As Stewart Wood noted: “A lot of the soft left are very pro-European. In fact, one of the key things that distinguishes them from the hard left historically is precisely the European issue. The soft left is much more comfortable with Corbyn’s domestic policy now, but because Brexit is the only issue in town for the next few years, it makes them much more concerned. That, for me, is the red warning sign for Corbyn.”

In office, the left has struggled to fulfil expectations. The most recent comparable socialist project, that of the former French president François Mitterrand, was brought to heel by the market within two years (resulting in the tournant de la rigueur, or “austerity turn”, of 1983).

“There is simply no historical model anywhere in the world for what we want to do, which has been successful,” a senior Labour insider told me. “A left government being elected in a post-industrial society and then successfully managing to transition into a major new settlement, whether a new form of capitalism or socialism: this is not easy to achieve.”

In his 2017 book The Great Leveller, a study of inequality since the Stone Age, the Stanford historian Walter Scheidel casts doubt on the ability of any domestic government to reduce inequality significantly without the aid of one of the four “great levellers”: mass warfare, violent revolution, state collapse and lethal pandemics. But McDonnell is defiant. “We went beyond the concept of socialism in one country many decades ago. To transform our society, we know we have to work internationally and globally.”

Having been so consistently underestimated, the left has earned the right to be taken seriously. “I would ask you to stay in the party and fight,” McDonnell told his supporters when he failed to make the ballot in 2007. He and Corbyn are reaping the rewards for this. Alternative left-wing projects such as George Galloway’s Respect, Arthur Scargill’s Socialist Labour Party and the Scottish Socialist Party have collapsed.

Once asked why he remained in the Labour Party throughout the Blair years, Jeremy Corbyn replied: “I remember discussing this with Tony Benn many times, and he said: ‘You know what, comrade, we’re just in it, aren’t we?’”

The night before he was defeated by Neil Kinnock in the 1988 Labour leadership election, Benn told a gathering of supporters: “I do not want anyone to think that tomorrow is the end. It is the beginning. It is twice as good as we thought it might be.” The following day, Benn won a mere 11.4 per cent of the vote. “It was appalling,” he recorded in his diary.

The new spirit of optimism on the left is perhaps best captured by Jon Lansman, who managed Benn’s failed 1981 deputy leadership bid. “After decades in the wilderness,” he said, “the promised land is not far off.”