The Second International, which was formed in 1889 and dissolved on the eve of World War I in 1914, consitituted some of the same parties that would later form the Socialist International. Among the Second International's most famous actions were its 1889 declaration of 1 May as International Labour Day and its 1910 declaration of 8 March as International Women's Day. While the Second International was split by the outbreak of World War I, a skeleton form survived through the International Socialist Commission. The International re-formed in 1923 (as the Labour and Socialist International), and was reconstituted again, in its present form, after World War II (during which many social democratic and socialist parties had been suppressed in Nazi-occupied Europe).
During the post-World War II period, the SI aided social democratic parties in re-establishing themselves when dictatorship gave way to democracy in Portugal (1974) and Spain (1975). Until its 1976 Geneva Congress, the Socialist International had few members outside Europe and no formal involvement with Latin America. In the 1980s, most SI parties gave their backing to the Nicaraguan Sandinistas (FSLN), whose left-wing government had incited enmity from the United States. Since then, the SI has admitted as member-parties not only the FSLN but also the centre-left Puerto Rican Independence Party, as well as the ex-Communist parties such as the Italian Democrats of the Left (Democratici di Sinistra (DS)) and the Front for the Liberation of Mozambique (FRELIMO).
The Party of European Socialists, a European political party active in the European Parliament, is an associated organization of the Socialist International. Recently many parties in the Socialist International have moved towards the center in order to gain more votes.
The Party of European Left is the more radical European version of this.