International Working Union of Socialist Parties

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, created by splinter groups of the Socialist Party of Romania). In April 1921, it was joined by the Spanish Socialist Workers' Party. The Maximalist faction of the Italian Socialist Party (PSI) also joined.

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 Sozialistischer Parteien ("News of IWUSP"). Poale Zion (labour Zionist) leaders David Ben-Gurion and Shlomo Kaplansky were active in the movement behind the Two and a Half International.

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