When American soldiers liberated the Mauthausen Nazi concentration camp in Austria 80 years ago this May, Spanish prisoners welcomed them with a message of antifascist solidarity. The Spaniards hung a banner made from stolen bed sheets over one of Mauthausen’s gates. In English, Spanish and Russian, it read: “The Spanish Antifascists Greet the Liberating Forces.” Both American servicemen and Spanish survivors remember the camp’s liberation as a win in their shared fight against extremism, my research on the Spanish prisoners in Mauthausen finds. They all understood the authoritarian governments of Nazi Germany, Italy and Spain as fascist regimes that used...