ABSTRACTThis article explores national dimensions of transnational interaction between the French Socialist Party (SFIO) and the German Social Democratic Party (SPD) in the 1940s–1950s within a comparative framework. Doing so allows us to uncover why the French and German parties retained intensive transnational contacts with one another despite their disappointments with postwar socialist internationalism. The SFIO and SPD were eager to put a socialist stamp on reconstruction, European integration, and French-German relations. The article shows why transnational engagement with their cross-Rhine colleagues appeared the most promising path to do so. Despite lengthy conflicts on Cold War politics, the international politics of reconstruction, and, later, decolonization and the Algerian and Suez Wars, the SFIO and SPD successfully compartmentalized their differences by establishing an autonomous track for transnational relations in European parliamentary assemblies. French-German cooperation on matters of European integration yielded the period’s greatest transnational successes, including the establishment of the Congress of Socialist Parties of the European Communities. In the early 1960s, the two-track nature of French-German inter-socialist relations endured but structural factors eroded the balance of power between French and Germans needed to sustain the mutual satisfaction with transnational cooperation that had peaked in the mid-1950s.