Short answers, pulled from the story.
The International Brigades were volunteer soldiers organized by the Communist International to assist the Popular Front government of the Second Spanish Republic during the Spanish Civil War, from 1936 to 1938. An estimated 32,000 volunteers from dozens of countries served, though no more than 18,000 were deployed at any single moment. Their headquarters was at the Gran Hotel in Albacete, Castilla-La Mancha.
Spanish Prime Minister Juan Negrín dissolved the International Brigades on the 23rd of September 1938, hoping the withdrawal of foreign volunteers would pressure the Nationalists' foreign backers, Germany and Italy, to also withdraw and persuade Western democracies to lift their arms embargo on the Republic. The gambit failed to achieve either goal.
Estimates of International Brigades deaths range widely depending on the source. An internal Soviet report from late July 1938 cited 3,615 killed; Prime Minister Juan Negrín mentioned 5,000 in his October 1938 farewell address; many scholars prefer a figure around 10,000; the highest scholarly estimate is 15,000. A precise calculation from the mid-1970s arrived at 9,934.
France sent the largest national contingent, with approximately 8,962 to 9,000 volunteers. Germany and Italy each contributed roughly 3,000 or more, though many were recruited in France and Belgium rather than their home countries. Poland also contributed a substantial contingent, with estimates ranging from 500 to 5,000.
Josep Almudéver, believed to be the last surviving veteran of the International Brigades, died on the 23rd of May 2021 at the age of 101. Born into a Spanish family, he held French citizenship and enlisted in the International Brigades to avoid age restrictions in the Spanish Republican army. He served in the CXXIX International Brigade and later fought in the Spanish Maquis.
Veterans faced vastly different treatment depending on their country. In East Germany, veterans dominated the postwar state's military and security apparatus. In the United States, returned volunteers were labeled premature anti-fascists by the FBI and pursued during the Red Scare of 1947-1957. Swiss volunteers were prosecuted in military courts, receiving sentences of up to four years in prison, and were not retroactively rehabilitated until March 2009.