International Socialism: Hegemony and revolutionary strategy The PSI was the only social democratic party in Western Europe to oppose the First World War and played a key role in organising the Zimmerwald Conference, held in September 1915 to rally the European anti‑war left. It was the first mass party to join the Communist International—the international grouping of parties, usually known as the Comintern, which supported the October Revolution. in Public bookmarkswith gramscihegemonyisjmarxismsocialism