This is a story that is far from over and is therefore much more difficult to put in perspective. Some issues – such as anti-Semitism and the leaked report from party HQ are the subject of internal and external enquiries and we need to wait for their conclusions. Nonetheless, it is possible to draw some tentative lessons from what has taken place.

The election of Corbyn in September 2015 can be seen at least in part as a reaction against the Blair/Brown years in office (1997 – 2010), which left many Labour members disillusioned with the Labour leadership. The Iraq war (and Blair’s support for President Bush) hung over the party. Ed Miliband (who had been narrowly elected after the left chose him, rather than his brother David) had failed to win the May 2015 election, seeing all but one of Labour’s MPs in Scotland lose their seats.

There was a general sense of depression inside Labour; a feeling that the party had run out of ideas and inspiring leaders. Against this background a small group of MPs and left wing activists met to select who should stand for party leader. Rather reluctantly Jeremy Corbyn recognised it was his turn to throw his hat into the ring. That is the established narrative.

A group of left-wing members came together under the leadership of Jon Lansman, who had joined the party in 1977 and who came into politics as an aide to the former left-wing MP, Tony Benn (who died in 2014). Momentum was founded to campaign for Corbyn, who struck a chord with members and succeeded in winning the party leadership in the first round.

Corbyn’s victory brought together a coalition of members working with a range of organisations. These ranged from the Stop the War Coalition (which Corbyn had chaired) and the Campaign for Labour Party Democracy (which Tony Benn had chaired) to trade unions, including Unite (formerly the Transport and General Workers’ Union). The Trade Unionist and Socialist Coalition was chaired by Dave Nellist - the former Labour MP expelled for his membership of Militant.

What was unique about Corbyn’s leadership of the party was that it took as an assumption that there were no enemies on the left. As a result, a number of far-left groups aligned themselves with the new leader. Trotskyist movements and Communist groupings worked together in ways that would have had been unthinkable just a few years earlier.

Exactly how this was organised will need careful research, but initially anyone could join Momentum. This included the Socialist Workers Party and the Alliance for Workers Liberty (formerly Socialist Organiser) which had previously been bitterly opposed to Labour.

More important were organisations aligned with the Communist Party. Two key figures stand out: Seumas Milne and Andrew Murray. Both had been associated with a monthly journal called Straight Left, which was associated with the “Stalinist”, pro-Soviet, anti-Eurocommunist faction that eventually split from the Communist Party of Great Britain. Murray, originally a Morning Star journalist, had been first chair of the Stop the War campaign and in 2011 was appointed chief of staff for the Unite trade union. He was seconded to work as one of Corbyn’s closest advisers. Milne had been a Guardian journalist; whose controversial pro-Putin and anti-European Union views caused many a row inside the paper. He became Corbyn’s director of communications. Together with Karie Murphy (Corbyn’s executive director) and Len McCluskey, (general secretary of Unite), these were the "Four Ms" who controlled the Corbyn leadership office.

They shared a deep distrust of the European Union (a view Corbyn had inherited from his mentor, Tony Benn) and a hostility to the United States and all that America stands for.  Corbyn’s half-hearted support for the Remain campaign was at least partly responsible for the failure of the party to mobilise its members in the 2016 EU referendum. The campaign was narrowly lost, with 51.89% of the public voting to leave.

While the Stalinists formed the praetorian guard round Corbyn, the Trotskyists were busy organising Momentum. At its height Momentum was a powerful organisation. It was entirely separate from the Labour Party, with a company Jon Lansman controlled privately owning the data of its members and supporters. It had its own staff and its own campaigns. Among these were the successful The World Transformed political festivals, held during Labour Party conferences. A number of media and social media houses backed Corbyn and Momentum. These included Skwawkbox, The Canary and Novara media. Together they were a powerful mobilising force, which perhaps saw their finest moment when thousands chanted Corbyn’s name at the 2017 Glastonbury festival.

Since the 2019 election, which saw Labour go down to its worst defeat since 1935, Momentum has suffered a series of splits. This does not mean that the far left are no longer a force inside the party, but their influence under Keir Starmer has certainly diminished.

Martin Plaut is a South African/British journalist and historian, who worked for the BBC World Service for 27 years. He is currently Senior Research Fellow at the Institute of Commonwealth Studies.

He tweets at @martinplaut