Second International

The Socialist International, commonly known as the Second International, was a political international of socialist and labour parties formed in Paris on 14 July 1889. At a time of growing industrial working-class movements and the expansion of suffrage, it brought together autonomous national parties into a loose international federation. It continued the work of the First International (1864–1876), from which it inherited both the legacy of Karl Marx and the conflict with anarchists. The organization was dominated by the powerful Social Democratic Party of Germany (SPD), whose organizational and theoretical leadership heavily influenced the other member parties.

The International established the annual celebration of International Workers' Day on 1 May and popularised the demand for an eight-hour day. Its early congresses were preoccupied with expelling anarchists and defining its mission as one based on parliamentary political action. After 1900, the International was increasingly confronted with the internal divisions of the socialist movement, particularly the rise of revisionism in Germany and the debate over socialist participation in "bourgeois" governments, sparked by the Millerand affair in France. The 1904 Amsterdam Congress, which saw a major debate between French socialist Jean Jaurès and German leader August Bebel, condemned revisionism and ministerialism, marking the highest point in the influence of the International.

From 1905, the prevention of war became the International's central task, and it came to be seen as the world's most important anti-militarist political force. At the congresses of Stuttgart (1907), Copenhagen (1910), and Basel (1912), it passed increasingly urgent resolutions calling for international working-class action, including strikes, to stop the outbreak of war. However, the International was powerless to stop the crisis of July 1914. Following the assassination of its most charismatic anti-war leader, Jaurès, its major member parties—including those in Germany, France, Austria, and Great Britain—rallied to support their respective nations' efforts in World War I, precipitating the International's collapse.

The schism between its pro-war majority and its anti-war minority, which organised the Zimmerwald Conference, prefigured the post-war split between social democracy and communism. Post-war attempts to revive the organisation at the Berne International conference of 1919 were unsuccessful, as many parties refused to rejoin what they saw as a discredited body. The Second International was succeeded by the Labour and Socialist International, formed in 1923 by a merger of the Berne International and the International Working Union of Socialist Parties, and the Communist International (Comintern).