Independent Social Democratic Party of Germany

The Independent Social Democratic Party of Germany (German: Unabhängige Sozialdemokratische Partei Deutschlands, USPD) was a short-lived, left-wing political party in Germany during the late German Empire and early Weimar Republic. It was established in 1917 by members of the Social Democratic Party of Germany (SPD) who opposed Germany's involvement in World War I. After much of the USPD's membership joined the Communist Party of Germany (KPD) in 1920, the remainder returned to the SPD in 1922, with the exception of an insignificant faction that lasted until 1931.

The Social Democratic Party officially supported Germany's entry into World War I in 1914, but a small number of members centering around Karl Liebknecht repeatedly voted against government requests for war bonds. The leadership of the SPD began expelling the opponents of the war in 1916, and in April 1917 a group of those who had been ousted formed the USPD. The new party, which included the far-left Spartacus League, agitated actively against the war, supported anti-war strikes and welcomed the Russian Revolution.

When the German revolution broke out in 1918, the SPD quickly took control of the revolutionary movement. They invited an equal number of USPD members to join them on the ruling Council of the People's Deputies, but friction between the two parties led the USPD members to resign after less than two months. The primary point of contention was that the USPD favoured a council republic controlled by workers' and soldiers' councils, whereas the SPD wanted a socialist-led parliamentary republic. The USPD participated in the Weimar National Assembly, which wrote the constitution for the Weimar Republic, although it voted against accepting the final document.

After the radical Spartacus League split from the USPD in January 1919 to form the Communist Party of Germany, tensions remained within the party over the extent to which it wanted to continue to push for a council republic. In 1920, a little over half of the USPD membership joined the KPD. Two years later, the almost all of the remaining USPD rejoined the SPD. A greatly shrunken USPD continued until 1931.