The Socialist International has its roots in the Second International, which was formed in 1889. Split by the outbreak of World War I, it was re-formed in 1923 (as the Labour and Socialist International), and reconstituted again (in its present form) after World War II (during which many socialist parties had been suppressed in Nazi-occupied Europe). Among the Second International's most famous actions were its (1889) declaration of 1st May as International Labour Day and its (1910) declaration of 8th March as International Women's Day.
During the post-World War II period, the SI aided social democratic parties in re-establishing themselves when dictatorship gave way to democracy in Spain and Portugal. In the 1980s, most SI parties gave their backing to the Nicaraguan Sandinistas (FSLN), whose leftwing government had incited enmity from the United States. Since then, the SI has admitted as member-parties not only the FSLN but also ex-Communist parties such as the Italian Party of the Democratic Left (Democratici di Sinistra) (PDS) and the Front for the Liberation of Mozambique (Frelimo).