One of the most famous revolutionaries in Irish history, Frank Ryan fought in the War of Independence and Irish Civil War, before becoming a leader of the inter-war Irish Republican Army. Embracing socialist republicanism in the 1930s, Ryan became a founding member of the short-lived but influential Republican Congress, and led the Irish contingent within the International Brigades during the Spanish Civil War. Following his capture by Franco's forces, his release was secured by the efforts of German military intelligence which sought to use him to further its wartime alliance with the IRA. He spent the final years of his life as an advisor to the Nazi regime. This study provides a concise reinterpretation of Ryan's controversial life and legacy based on primary sources, including much previously unpublished material. Setting Ryan's career within the wider context of the violent politics of inter-war Europe, it examines his remarkable ideological journey from socialist republicanism to Nazi collaboration. It concludes by examining why - despite his lack of political success - Ryan became such a widely admired figure in modern Irish politics.