The Italian Socialist Party represents an anomaly within the European family of Socialism, which, in 1947, at the beginning of the Cold War, made a clear choice for the West. On the contrary, the PSI refused to break the link with the Italian Communist Party, although this loyalty would cause for the rejection from the International Socialist Organization (COMISCO) and the rupture of the Party’s unity. Italian Socialism, divided in two parties (PSI and PSDI), became a weaker political force in the Italian political system, dominated by the Catholics (DC) and Communists’ (PCI) big organizations. At the beginning of the Fifties, and in particular in 1956, in order to regain a central role in the Italian political scene, Pietro Nenni, leader of the PSI, attempted to reach an agreement with Giuseppe Saragat, leader of the Democratic Socialist Party. Based on documents available in international archives such as the International Socialist Organization Archives in Amsterdam and the Archives of the Labour Party in London this essay analyses, step by step, the rupture with the COMISCO (1947-1949) and the attempts to rebuild the unity of the party and to re-enter the ISO (1955-1963).