You’ve asked quite a few questions there! I’ll do my best to break them down and give a concise-ish answer to each, but we’ll see how many pages I end up needing…

First, regarding the role of communists in Republican Spain. Communism plays quite an interesting – and controversial – role in the outbreak and course of the Spanish Civil War. Before the war, the Communist Party was a significant but not massive force in Spanish politics. They had by 1936 recovered somewhat from their nadir in the early 1930s, but were far from the largest or most significant political party of the left. While they were a key driving force behind the electoral pact known as the ‘Popular Front’, which allowed a coalition of leftist parties to win power in the February 1936 elections, the communists were reluctant to participate directly in this new government. This reluctance stemmed partly from weakness – they were still a relatively minor party (perhaps 30,000 members and 15 or so seats in parliament after the 1936 elections) – but also because they acknowledged that their direct participation might discredit the new Popular Front government, whose main figures were initially ‘Left Republicans’ (ie liberals, broadly speaking) and more moderate socialists. While it might seem strange for communists to be so circumspect, this was in line with broader international communist policy, which emphasised the building of such ‘Popular Fronts’ as a reaction to the rise of Nazism in Germany. The communists had realised that divisions on the left, particularly between German socialists and communists, had allowed Hitler to gain power and prevented them from opposing his rule until it was too late. Communists had initially expected that Hitler was the last, desperate gasp of capitalism, which would soon collapse and give them their opening for revolution. Instead, as we all know, Hitler took power and moved to stamp out political enemies, starting with the left wing political organisations. The communists realised that their priority needed to be preventing fascist governments coming to power, rather than plot their own revolutions, and the best way to do this was promote left-wing unity against fascism.

This broad policy – promoting unity against fascism – was to inform the Spanish Communist Party’s approach throughout the Spanish Civil War, trying to dampen revolutionary activity in favour of prosecuting the war against the military uprising. This attitude was famously deprecated by George Orwell in Homage to Catalonia, who argued that the communists had thwarted the revolutionary desires of the Spanish people on instruction from Stalin and the Soviet Union. It’s worth noting that Spain was also home to the world’s largest anarchist movement – there were far more anarchists than communists in Spain in 1936 – and particularly in regions such as Catalonia, there were competing visions of what the war should be about. There’s a number of existing answers (including my own attempt here, or this earlier one) touching on this enduring controversy, so I won’t go into it here.

However, while the Communist Party of Spain was still a minor political force on the outbreak of war, the war itself saw them grow in strength considerably. War offered the communists several advantages that they leveraged to expand their influence and membership. Through the Communist International (Comintern), they had a network of international contacts that was better organised and resourced than any other grouping. In particular, this meant they had strong connections to the Soviet Union, who soon proved to be one of the few countries willing to support the Republic directly by supplying arms, supplies and advisors. This naturally gave the Soviet Union increased prestige and support within Republican Spain (although to be clear, this can be overstated – they had influence but not direct control). The Communist Party was also well placed to contribute to the war effort directly and thereby gain standing as particularly effective defenders of the Republic. Many of the early militias that were formed as a reaction to the coup attempt in July 1936 were based along political lines – supporters of particular parties or trade unions would band together locally to fight the military uprising. Even after the Republican Army was regularised into standard divisions and brigades, in practice individual units were usually still dominated by one political grouping (so as well as communist brigades, there were socialist units, anarchist units and so on). Communist units tended to be particularly effective – the Party’s emphasis on internal discipline translated well to a military context, especially compared to the more chaotic anarchist approach. In fact, the communist obsession with discipline won it support among the remaining loyal military officer corps, who were frustrated by what they saw as the lack of discipline in many Republican units. This meant that over time, communist units were generally more disciplined and better led than average, and their influence within the military hierarchy grew – particularly as they were also indirectly the source of many of the weapons from the Soviet Union. So, by the end of the war, the communists were much stronger than they had been at the start, but were still far from a majority of what was still a very varied Republican support base.

But if we move to the second part of your question – the international volunteers – this picture is quite different. As you note, there were many such volunteers from many countries on both sides, and they were naturally far from homogenous. Yet discounting the ‘volunteers’ sent to Spain by Hitler and Mussolini, who were usually just regular soldiers, the bulk of foreign volunteers in Spain were communists or their supporters. Most foreign volunteers came as part of what were known as the International Brigades, an international army organised by the Comintern as part of the Soviet aid for the Republic – most of its higher leadership were Comintern operatives. As most smaller units were organised by nationality, these were generally led by members of the relevant Communist Party (eg the CPUSA for the American Lincoln Battalion). All told, about 35,000 foreigners served in these units between October 1936 and September 1938 when they were withdrawn. Approximately 60% of them (with some variance among national groups) were actually members of their national Communist Party. Even among the others, though, I would argue that the bulk came from a similar political background. Among the British volunteers, for instance, many were instead members of the National Unemployed Workers Movement, which was essentially a communist front organisation. Overall, I think it’s fair to say that the best overarching description of the volunteers is ‘anti-fascists’, and included some liberals, socialists and others who were not communists – but almost all still tended to agree with the Communist Party about how best to oppose fascism, even if they didn’t subscribe to all of their political beliefs. It’s worth noting though that there was in the 1930s no neat dividing line between ‘democrats’ (small ‘d’) and ‘communists’ – many adhered to communism because they saw it as more rather than less democratic than their home societies. While obviously in retrospect they misjudged the Soviet Union, most were not cynical in their beliefs, and many went on to highlight later that they went to Spain to try and defend democracy rather than usher in a socialist revolution.

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