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The International Working Union of Socialist Parties (IWUSP; also known as 2½ International or the Vienna International; German: Internationale Arbeitsgemeinschaft Sozialistischer Parteien, IASP) was a political international for the co-operation of socialist parties. IWUSP was founded on February 27, 1921 at a conference in Vienna, Austria by ten parties, including the Independent Social Democratic Party of Germany (USPD), the French Section of the Workers' International (SFIO), the Independent Labour Party (ILP), the Social Democratic Party of Switzerland (SPS), the Social Democratic Party of Austria (SPÖ), and the Federation of Romanian Socialist Parties (FPSR, successor to the Socialist Party of Romania). In April 1921, it was joined by the Spanish Socialist Workers' Party.

The secretary of IWUSP was the Austrian Friedrich Adler of the SPÖ; other prominent members were Victor Adler, Otto Bauer and Julius Martov. The group was heavily influenced by Austromarxism. It published Nachrichten der Internationalen Arbeitsgemeinschaft Sozialistischen Parteien ("News of IWUSP").

The founders of IWUSP were parties that saw neither the reformist Second International nor the communist and pro-Soviet Third International as alternatives for affiliation. The IWUSP criticized the other two Internationals for what it perceived to be dogmatism, and meant that more consideration should be given to the particularities of the political situation in each country. It worked for the unification of the Second and Third Internationals. From April 2 to April 5, 1922 a meeting was held in Berlin with delegations from the three different international bodies to discuss a merger, but unity could not be achieved and the Comintern withdrew from the talks.

In Germany on September 24, 1922, the USPD, one of the main components of IWUSP, merged with the Social Democratic Party of Germany (SPD), a member of the Second International. Discouraged by the intransigent position of the Third International, at the joint congress with the Second International held in Hamburg in May 1923 IWUSP merged with it to form the Labour and Socialist International. Some, such as the FPSR, refused to join the new body.

In the 1930s, a similar effort was made to create an international between the reformism of the Second and the Stalinism of the Third, as the London Bureau of left-wing socialist parties. Sometimes called the "Three-and-a-Half International", it involved many of the same parties.

• Archive of the International Working Union of Socialist Parties
• A Communist view on the Conference of the Three Internationals

• Lenin "The restoration of the International"
• Trotskyism versus Centrism in Britain

• Democratic socialism
• Berne International