Shortly after Corbyn first won the leadership election, I wrote an article for Le Monde Diplomatique, trying to explain how something that nobody thought was possible had happened. The book is an attempt to build on that and show how, despite the great surprise, Corbyn’s victory did not come from nowhere, but from a number of discernible trends that had been developing for some time. I had a good range of contacts due to my time writing for Red Pepper, so that gave me access to a lot of the inside voices.

Telling the story of how Corbyn won is important. When you’re involved in a big political struggle, if you understand its history then you’re much more able to navigate it successfully. I hope my account can help people enthused by Corbyn to understand the terrain they’re fighting on, as well as explain events to some of those who have so far simply dismissed Corbynism as some kind of aberration.

There were three major strands which came together in Corbyn’s first leadership campaign, and which continue to form the basis of the movement around him. These were existing Labour Party members, the trade unions, and a range of people on the Left without party affiliation, many of whom were active in various social movements. These people became the £3 members. What all three stands had in common was that they all sprang from resistance to the Thatcherite economic model, or neoliberalism, which suffered such a catastrophic collapse in 2008 and hasn’t recovered.

One thing I try to show is that the last of these three elements, the £3 voters — as important as they were — only really came into play after developments in the first two strands had put Corbyn in a strong position. In other words, the Labour Party was not simply “taken over” by outsiders, as you might think from most of the media coverage. There were long-term developments within the labor movement that, in retrospect, can be seen as crucial to Corbyn’s victory.

Contrary to popular perception, it was existing Labour members — hugely disillusioned with the New Labour project — who put Corbyn in the lead in the first few weeks of the 2015 contest, before the broader anti-austerity movement got behind him and flooded in. As I detail in the book, canvassing data clearly support this timeline.

In parallel to this, the unions began to respond to the end of their long-standing alliance with the Labour right. Eroded by several decades of neoliberal restructuring, this influence was further reduced under Tony Blair. Why bother with the unions? They were seen as an old-fashioned, declining force. As far as Blair was concerned, they were simply an impediment, and he was aggressive towards the unions during his time in charge. Most of Thatcher’s anti-union laws remained in force; Blairites talked of severing the party’s link with the unions; leading trade unionists were briefed against in the press.

The unions therefore found themselves as an opposition within their own party, shut out of their traditional alliance with the Labour right. This was new for them, and they had to figure out how to exert influence. There was also discontent among the rank and file, as members increasingly began asking what the point of affiliating to Labour was, when the party gave them nothing and indeed regularly humiliated them.

Given these developments, the unions began to fight for their influence within Labour, forming a series of tactical alliances with elements of the Labour left. It became clear how big an obstacle the Parliamentary Labour Party (PLP) was, and so a number of unions openly developed political strategies to try and reshape it in their favor. For example, Britain’s biggest union, Unite, had an official policy of supporting working-class (but not necessarily left-wing) parliamentary candidates, and political schools were organized by various unions to train potential MPs. The hope was that by 2020 they’d have a candidate for leader, with good support in the PLP.

This inevitably brought them into conflict with the Labour right, and in particular the Blairites, most notably over the so-called “Falkirk controversy”; Unite was accused of trying to stitch up the selection process for a friend of Len McCluskey, their general secretary. Although it later turned out Unite had not broken any rules, these allegations set off a bizarre series of events and provoked quite spectacular miscalculations from the Labour right, all of which — via a circuitous route — culminated in a radical democratization of how the party elects its leaders. Under a huge amount of pressure, the unions gave up their third of the leadership vote over who leads the part that had been guaranteed to them under the old electoral college system, on the condition that the PLP did the same.

The result was the switch to a one-member-one-vote system (OMOV) and the opening up to the public of the £3 voting fee. Ironically, this was driven by the right in the belief that it would finally smash the influence of the unions and the left — the result, of course, was very different. That Ed Miliband, the supposedly pro-union Labour leader, sided with the right during this episode, meant that union leaders were more open to something they would have never considered before: backing a figure from the radical left, more out of desperation than any expectation of actually winning.

Absolutely. Although the union leaderships have so far generally been solid in their public support of Corbyn, most notably during the coup attempt, they also represent his biggest vulnerability. When, for example, the National Executive Committee (NEC) met to decide whether Corbyn would be on the ballot for the second leadership election, it was only the votes of the trade unions that got him on. This illustrates their constitutional power within the Labour Party and how crucial their support is — consequently, if that support shifts for some reason, he could be in trouble.

In terms of looking for cracks, my feeling is that the Labour right is beginning to recognize this, and understand that they need to organize themselves better within the unions. Until now, they have ignored this, perhaps due to complacency — remarkably, they didn’t consult the unions before mounting the coup. But I think the right’s involvement in the unions is something that we’ll see more and more of in the coming years.

Whether or not they’ll be successful is another question. But it’s certainly true that the support of unions for Corbyn is not set in stone. It is contingent, and a potential vulnerability.

There were various ways in which the Blairite project broke down. First of all, we shouldn’t conflate Blairites proper with the right of the PLP. The former have always actually been a fairly small group, at least in terms of the “true believers.” Indeed, Blair himself once said that there were actually only five members of New Labour. So they’ve always been a small clique who controlled Labour through the party machine and their occupation of influential positions, rather than numerical strength.

This meant that there was always a potential fragility, even if this was invisible for a long time. If you don’t actually have a base in society, then you always risk being swept away when things change.

There’s another factor as well. The quality of Labour MPs declined in the Blair period. Because the New Labour project put such an emphasis on shutting out the Left and ensuring that their preferred candidates got selected to Parliament, conformity was prized over talent. So Progress [a Blairite pressure group within Labour] would take promising-looking people from student politics, give them roles working for a right-wing MP, and gradually move them through the system — as councillors, special advisers, MPs, and eventually cabinet members.

The cumulative result was a parliamentary party full of people who had never had to struggle for their political philosophy or fight for their positions. Their abilities became increasingly limited, until they lacked anything like a political worldview that would allow them to adapt when confronted with a new situation. They lack big thinkers or any kind of real ideology beyond a certain technical competence.

There are two big tensions which keep recurring. One of them is about whether Momentum is a social movement or an internal Labour faction, a sort of “Progress of the left.”

At the moment it’s trying to be both, and this is a difficult balance to strike. The social movement aspect is clearly very important and is a large part of what motivates people about Corbyn’s leadership, but we have also seen the need for a dedicated Corbynite faction within Labour. It was remarkable, for example, that after Corbyn won an absolute landslide victory in the second leadership contest, he still lost motions on the conference floor, with the right boosting their strength on the crucial NEC. This shows the deficiencies of the left’s organizations in these areas compared to the right, who have been doing this for a long time.

So while we can have debates about whether the emphasis should be on building a social movement or taking over the party internally, the problem is that they really have to do both.

The second tension is perhaps less widely understood. It’s more of a cultural division, between people who grew up in the more traditional labor movement and those who come from the anti-austerity movements, the alter-globalization movements, and so on, and who tend to organize in a different, much more horizontalist way.

Whether those two can get on and figure out how to progress is the big unresolved question of Momentum. We saw this tension flare up recently with arguments over whether Momentum should have a conference with delegates to decide its positions and policies, or whether it should have a kind of digital conference based on OMOV. Generally speaking, people from the traditional labor left supported the former position, and those from the social movements the latter. At the moment, it seems there will be an attempt to resolve this with a hybrid system.

That this tension is playing itself out inside Momentum is a consequence of the fact that for a long time the Left was at a weak point, and in many respects still is. This is all brand new, and this movement doesn’t have established ways of organizing things or settled power structures. It’s all up for grabs, and that’s quite exciting in a way.

The issue you raise is a real one, and is a historic problem. The social movement aspect of Corbynism is probably its most exciting and dynamic part, but at the same time this kind of struggle was finding it hard to make lasting gains after 2008, in Britain and elsewhere. Occupy is probably the best example of this. In flooding in to support Corybn, people involved in these movements are searching for an electoral path to continue the work that they’d been doing in a different arena. In that endeavor, they’ve found themselves allied with the traditional labor movement, as I’ve said.

So far it’s worked, especially when there have been clear goals such as getting Corbyn elected and re-elected. Whether it will continue to work is another question — I do think it’s quite an unstable alliance.

If Corbyn is removed, or if he loses an election and the internal battles within the Labour Party haven’t been won, then these instabilities will, I think, be quite starkly revealed, although I couldn’t predict where things would go. Given that, I think it’s probably better for Corbyn if the election is not called early, but is held in 2020. They need time to build.

However, we can’t let this possibility of the whole thing falling apart paralyze us. The Left is weak, true — but maybe not as weak as we sometimes imagine ourselves to be. We always seem to be on the defensive. Now more than ever, we need to have the confidence to present a dynamic alternative, both in terms of policy and a popular message.

There are obviously ideological and institutional obstacles to that, but building this confidence and dynamism between now and the next election is key.

Part of this also involves participating in struggles not directly connected to the Labour Party or electoral politics, and that’s one of the great benefits of having Corbyn as leader. He’s campaigned with social movements for decades, and he’s never demanded to see people’s membership cards before building common causes. It’s important that these struggles continue, as arguments around foreign policy, health care, austerity and so on have to be won in society, rather than the purely legislative solutions being sought in Parliament.

After Brexit and Trump, you can no longer say that Corbyn was just a strange phenomenon within the Labour Party that doesn’t require some sort of wider explanation. Politics is changing and extraordinary things are happening all over the world. Corbyn is one manifestation of this; Trump is another.

Although the right-wing solutions are more prominent and dominant at present, the Left has to offer a relevant analysis and solid answers. This doesn’t mean that there won’t be tensions within and between the various sections drawn to Corbyn. But there are countless issues around which different groups could mobilize, both inside and outside of Labour and Momentum. Building those links through practical struggle and active campaigning is not easy, but that is how progress has always been made in the past, and will continue to be made in the future.