I’ve read that American volunteers (the Abraham Lincoln Brigade) were commonly discriminated against and while many joined the U.S Army for WW2 but were denied officer positions due to their Communist alignment. If American volunteers were treated like this in the U.S what was life like for the German and Italians that fought for the Republicans in Spain. I’ve heard of the famous Hans Beimler, a German Communist Party member who fled Dachau concentration camp and later joined the International brigades. Was it more common for Italians to join the Partisan movement and the Germans to join the isolated groups that existed, were they generally punished severely by the Fascist governments of their home countries, or was it more common to try and assimilate back into society.
This is a fantastic question, which I sadly don't have time to address in depth - here is the short and dirty version, focusing on Germany as I have a relevant text to hand. Italians had a similar trajectory, though I believe they played a more prominent role in domestic resistance/partisan movements during the war.
The decision was taken to withdraw the International Brigade volunteers in late September 1938, the Republican Government agreeing to do so in the vain hope of putting political pressure on the Italian Government to withdraw direct support from Franco (not to mention that the International Brigades, now completely starved of foreign reinforcements, were no longer especially effective). The c.10,000 remaining foreigners were withdrawn from the front shortly after, and there was a large, emotional farewell parade in Barcelona in mid-October. About half of the foreign volunteers, though, had nowhere to go from Spain – their governments were either actively hostile or had no desire to take them back. This meant that even as the war entered its final phase, there were still a few thousand foreign volunteers left in Spain. They were lucky in one sense, however – the International Brigades had been concentrated in Catalonia ever since Republican territory had been divided in two by the Nationalists’ Aragon offensive in March-April 1938. This placed them close to the French border, offering a chance at escape if needed.
This prospect soon became a reality. Many of them joined in the final fighting retreat to the French border during the fall of Catalonia in early 1939, crossing into France alongside hundreds of thousands of Republican refugees and soldiers. The French Government didn’t really know what to do with these masses of refugees, and ended up interning them in makeshift concentration camps, mostly in the south of France. There were various schemes to get the volunteers and other refugees out of the camps – Mexico, for instance, was quite willing to take Republican refugees, but arranging passage was expensive, and only a small fraction could be sent over the Atlantic. Controversially, the Soviet Union was unwilling to let many ex-volunteers come there, even though most were communists. Most commonly, ex-volunteers languished in French concentration camps until the onset of the Second World War. Many escaped only with the Fall of France and many sought to join Allied armies or to hide, while others ended up in Auschwitz or other Nazi camps. Wherever they went, they tended to become key players in resistance and partisan movements – not only did they have considerable military experience, communists were often in the best position to organise this kind of activity as party structures were designed to operate underground. Spanish veterans, therefore, tended to have the knowledge and connections to be given trust and responsibility in this new context.
This meant that German volunteers were scattered everywhere during the war. Of the East Germans, about whom more is known due to the commemoration efforts of the postwar DDR, bout a third ended up in captivity in Germany during the Second World War, generally being transferred from those same French camps where they were stuck when France fell in 1940. Smaller but significant numbers (maybe 15-17% apiece) ended up in the French Resistance or made it to the Soviet Union. Of those who managed to flee, the most significant numbers ended up in Switzerland, Mexico and Sweden, which each took about 2% of the total. Others were scattered piecemeal around Europe and Latin America. About 3% remained in Spanish Nationalist prisoners of war camps.
These numbers are from Josie McLellan, Antifascism and Memory in East Germany (Oxford, 2007).