It is indeed fascinating how the whole world was looking at Spain at this moment in history - it's exactly what sucked me into the topic checks notes ...many years ago. The international dimensions of the Spanish Civil War fascinated people at the time and ever since, and so there is, in theory, a huge amount of scholarship on the ways in which Italians, Russians, Albanians and many other nationalities ended up fighting in Spain.

But despite all these decades of history writing, you've actually asked a very specific question that isn't actually answered all that well by historians - that is, why so many. We know a huge amount about the motives of volunteers, the various personal and political factors that made individuals want to go and risk their lives in Spain, but historians have actually not been great at explaining why Spain proved so uniquely attractive for foreign volunteers in the first place - in modern history, only the recent wars in Syria and Iraq have produced comparable numbers of what are often known as 'foreign fighters' (defined by Nir Arielli as individuals who fight in wars without being sent by their own government, and not primarily for monetary gain). While small numbers of such individuals are actually relatively common across contexts, it's very, very rare that more than a few hundred get involved in any given conflict, let alone the tens of thousands that fought in Spain.

As I mentioned, historians have paid a great deal of attention to the personal motives of volunteers. This question has always heard a certain fascination - their motives are distinct from the usual pressures to take up arms, they were not defending their homes and families, they did not stand to gain financially nor did they owe Republican Spain any personal loyalty. This, rather logically, meant that understanding these beliefs has always dominated academic studies of the foreign volunteers in Spain. Debate over these beliefs dominated a lot of Cold War-era scholarship in particular, with two broad schools of thought emerging: what I think of as ‘celebratory’ and ‘revisionist’ approaches, which even with the end of the Cold War tended to dominate historical scholarship until roughly the last decade or so.

Celebratory approaches – often written by those who had gone to Spain, or with a close personal connection to the conflict – highlighted the extent that the volunteers were motivated by the desire to oppose fascism. Many, such as those living in exile from Germany and Italy, had experienced fascism first hand, and saw the chance to continue the fight in Spain. Others were internationalists, who saw the expansion of fascism abroad (and the growth of domestic fascist movements at home) as a threat that crossed national borders – nowhere would be safe so long as fascism was in power. Although the volunteers came to become anti-fascists in different ways, they shared this common belief that it was a menace that had to be fought sooner or later – either today in Madrid, or tomorrow at home. Spain, in this worldview, was readily appreciated as the key flashpoint of the anti-fascist struggle at the time – as nation after nation succumbed weakly to fascism and dictatorship in the 1930s, Spain seemed to show that if fascism were resisted, a united effort could still hope to defeat it. Fascism, in other words, no longer looked inevitable or invincible in Spain. As such, for those keenest to fight against it, Spain was the logical place to go.

In contrast, revisionist accounts were a bit skeptical of this idealistic explanation. Without impugning the motives of every volunteer, they pointed out that significant numbers had less ‘pure’ reasons to be in Spain – some were bored, or unemployed, seeking adventure or money. This, as more source material has become available, has not stood up all that well – while a small minority of volunteers certainly fell into these kinds of category, most did not, and in any case there was no reason that someone couldn’t be, say unemployed AND a staunch anti-fascist. The more fundamental part of the revisionist case, however, was political: that while ordinary volunteers might have been idealistic, the project itself was not. The International Brigades, the units in which about 90% of Republican foreign volunteers served, were recruited, organised and led by the Communist International (Comintern), as part of wider Soviet efforts to support the Republican cause. This, in the words of one early revisionist scholar, made them a ‘Comintern Army’, doing Stalin’s bidding – and, in the eyes of a good Cold Warrior, this was an inherently Bad Thing. What revisionist accounts did do, however, was explore many of the less nice sides of foreign volunteering – Stalinist paranoia, political repression, executions, desertions and so on. But they did not manage to really shift the debate surrounding your question very far – that is, why did people volunteer in the first place?

More recently, scholarship has taken a transnational turn, emphasising how Spain was one part of a broader world in which activists and exiles lived and moved across borders and between struggles. These accounts place more emphasis in the place of Spain in wider life trajectories, including highlighting the role that Spanish veterans played in ant-fascist resistance across Europe after the civil war was over. Much of this scholarship is rather biographical, seeking to understand individual decisions in more depth, highlighting a complex range of push (ie reasons to leave home) and pull (reasons to go to Spain) factors that were at play for individual decisions. This has led to more nuanced explanations of these decisions, that do a fair bit to reconcile the competing narratives of celebratory/revisionist scholars, acknowledging both the reality that most volunteers were motivated by anti-fascist beliefs, as well as revisionist skepticism that this was the whole, unvarnished picture by acknowledging circumstantial and personal reasons for individual decisions. However, I personally have problems with this approach. These biographical approaches tend to overemphasise individuals who led interestingly transnational lives. Many, if not most, volunteers were not romantic exiles lurking in bars in fashionably bohemian districts of Paris, but relatively ordinary, working-class individuals who had never been particularly mobile before going to Spain. This is a problem when we want to talk about the bigger picture – it’s wonderful to have such a deep appreciation of a handful of volunteers, but if they aren't necessarily representative of all the volunteers, your original question is still unanswered - if nothing else, why were there so many anti-fascists lurking in bars in Montmartre for whom Spain represented a real and enticing prospect?