Red Missionaries explores the communication strategies of French and German Socialist parties during the inter-war period (1920-1939). With the help of eighteen newspapers – ten Bavarian and eight Breton – alongside a rich variety of both primary and secondary sources, this book examines how the Section française de l’Internationale ouvriere (SFIO) and the Sozialdemokratische Partei Deutschlands (SPD) navigated political messaging in two traditionally conservative regions. Focusing on electoral campaigns and May Day-related events between 1919 and 1939, the study challenges the prevailing view of these parties as inflexible and doctrinaire. Instead, it reveals a surprising adaptability and nuance in their approaches, questioning the notion that both parties inevitably became more bureaucratic and conservative during this period. Through detailed analysis of press coverage and party activities, Red Missionaries uncovers unexpected similarities between the SFIO and SPD, suggesting that their strategies were more aligned than previously assumed. Ultimately, this book not only reconsiders established narratives but also deepens our understanding of socialist politics in inter-war Europe.