The ILP is indeed a bit of an oddity in British political history. To understand it, you have to leave behind some of your assumptions about modern political parties. The UK Labour Party did not spring into existence (as we would recognise it at least) in 1906, still less in 1900 with the founding of the Labour Representation Committee. In its early decades, it’s much better understood as a federal body, a broad tent encompassing a wide range of socialist, trade union and other leftist/working-class organisations, with little substantive existence beyond the sum total of its affiliates. The ILP predated both of these overarching systems (it was one of the key early participants in the LRC, alongside groups like the Fabians). You could even argue that the ILP was the key early affiliate of the emerging Labour Party – its members (like Ramsay MacDonald and Keir Hardy) were among the most prominent and successful Labour politicians at the turn of the twentieth century.

The ILP certainly shared similarities to the Labour Party in structural terms (not least in that its ideological programme was vague enough to attract support from very disparate interest groups), but it still made little sense to disband completely in the context of the early Labour Party. The ILP represented a key interest group within Labour’s structures, and being an affiliate allowed ILP members to retain significant influence over the emerging party’s policies and decision-making while retaining significant freedom of action. This was how early Labour politics worked – stuff like choosing candidates, deciding on policy platforms and party leadership was a constant negotiation between different internal interest groups that retained their own independent structures and memberships. Indeed, the Labour Party didn’t have any notion of even having its own members before 1918 – you could only join by joining an affiliate organisation like the ILP.

This meant that the ILP itself continued to grow and expand within Labour up to the 1920s, and it became a natural home for many of the more radical activists within Labour, though this also meant dangers of splitting (notably due to the question of supporting the war effort in 1914, and over affiliation to the Third International in 1920-21). These issues showed some important fraying around the edges of the ILP as an organisation – on one hand, it was one of the (if not the) most dynamic, creative and influential socialist institutions in Britain, but on the other, its lack of a cohesive ideological identity left it trapped between Labour’s reformist tendencies and the more radical revolutionary positions that emerged in the wake of the 1917 Russian Revolution (and who looked more directly to the Bolsheviks for inspiration). Particularly as Labour achieved greater mainstream political success in the 1920s, these tensions grew more acute – it was one thing to make vague, aspirational promises as political outsiders, but actually forming government (as Labour eventually did in 1924) inevitably meant disappointing the more radical dreamers in the movement. Despite the ILP’s nominally large influence at the top of Labour politics (a significant proportion of MPs were affiliated all the way up until the 1932 disaffiliation), there was constant tension between its core membership (which tended to be quite independently and locally minded) and Labour’s leadership.

The result was that the late 1920s saw increasing divergence between the ILP and Labour, as the former started pushing a more concrete set of policy proposals, which went far beyond what the more mainstream Labour leaders (particularly the more socially conservative trade union leaders, who formed an increasingly powerful voting bloc within the Labour Party) were willing to consider. The result was that the ILP increasingly took on the character of the internal left opposition to the mainstream leadership (a recurring feature of the British Labour Party in various forms even after the ILP’s demise). Especially as ILP-affiliated MPs grew less influential at cabinet level, the question of how far they should continue to unquestioningly support the Labour Party line grew more acute, and by the early 1930s disaffiliation was being openly debated. ILP candidates stood independently of Labour in the 1931 election, and the 1932 ILP conference decided on formal disaffiliation the next year, forcing its members (including MPs) to choose sides.

A small parliamentary group (under the leadership of James Maxton, one of Labour’s more prominent and well-liked political figures in Parliament) survived this process and the subsequent General Election in 1935. These MPs all represented Glasgow electorates, which was the last remaining stronghold for the Party (and even here, long, ugly battles over ILP assets were fought with those who remained in Labour and formed a new affiliate, the Scottish Socialist Party). The ILP basically became the principled opposition party to just about everyone – they survived due to the residual goodwill and grassroots connections in Glasgow, combined with their ability to speak independently on national and international issues as a parliamentary grouping. They had a tendency to criticise just about everyone from Tories to Communists, but were no longer in a position to really achieve much by it.

Their response to the Spanish Civil War typified this. They were among the most ardently pro-Republican political groups in Britain, and worked throughout the war to raise money, awareness and even a small number of volunteers to fight in Spain. They initially agreed to support a United Front with other radical pro-Republican groups such as the Socialist League and the Communist Party of Great Britain, but their insistence on standing by their sister party in Spain (the POUM – best known as the group George Orwell fought for and described at length in Homage to Catalonia) led to an ugly break with the CPGB, who backed the central Popular Front government in cracking down on disruptive ‘Trotskyist’ elements in Barcelona. The ILP grew increasingly isolated as their radical policies grew increasingly out of line with the Labour Party, while their principled anti-Stalinism won them few friends on the radical left in the 1930s-40s. As such, while they survived in parliament for a while longer (the postponed elections due to World War Two helped in this regard), by the end of the war they were on their last legs, especially once Maxton died in 1946 and their last MPs defected back to Labour in 1947. They eventually rebranded as the ‘Independent Labour Press’ but to the best of my knowledge, never really regained much in the way of prominence or influence.

The ILP was hardly unconscious of the cost of disaffiliation, and discussed trying to reaffiliate on multiple occasions. In 1939, they even approached Labour with an offer to come back, with the proviso that ILP MPs be allowed, essentially, a conscience vote on key matters where ILP and Labour policy diverged. But the harsh electoral world of the 1930s had done a lot to reshape Labour – this was the decade that saw the emergence of a true central Party bureaucracy and national branch system, with the Trades Union Congress emerging as a parallel but increasingly distinct centre of influence in British politics. Neither the trades unions nor the strengthening Labour Party machine had any interest in these kinds of compromises any more - the political and intellectual space from which the ILP had emerged had already ceased to exist. The result was a more modern Labour Party better capable of winning power and maintaining internal discipline, but also one which had sacrificed much of its founding identity.